
In response, I have another wall of text for you. 
1. What's the best way to respond if someone meta-games?
There's a whole bunch of options here depending on how invested you are in the RP. For minor things -- knowing your name before being introduced, identifying your class when you've no visible indication of it, and the like -- my recommendation is to come up with an IC way they could have known that and roll with it. These sorts of minor infractions usually come from newer RPers and aren't really worth a great deal of concern. If you like, at the end of the scene, you might send the player a tell OOC thanking them for the RP and commenting that, as a bit of advice, they may want to avoid taking that sort of OOC knowledge IC.
For more serious infractions -- such as knowing private background information off your wiki page or your 100 Questions, for instance -- I think the best first step is to immediately send the player a tell OOC and, being friendly (naturally), ask them how they came across that information ICly. If you don't really mind them knowing it, you can offer some IC options by which they could have come across it, but make it clear that you'd prefer they find that information out ICly in the future. If it bothers you, or if you want to draw a line in the sand, you can offer a quick retcon of that part of the scene. That's one of the reasons why you have to react quickly; it's a lot easier for someone to say, "Okay, ignore that last thing I said," than to say, "Okay, ignore that whole last conversation we had."
If the player ends up being belligerent about it, or if the infraction is egregious (such as the player writing themselves into your background or knowing a deep secret of yours), just walk away. You always have the option of taking your ball and going home, so to speak. Respectfully tell the player OOCly why you're doing this, then inform them that you don't feel comfortable continuing the scene. If you and the other player share an IC LS or FC, you should also let the leadership know what happened; many such groups have policies about metagaming and may want to have a word with the player in question, especially if they're being awful about it.
2. For planned RP sessions, I've seen some of you said they're usually just simple "Let's go x and do y," but how exactly are more storyline RP scenes planned out?
This varies from person to person and from group to group. Some groups like to have a sequence of "closed" (i.e., not open to other RPers) scenes planned out, including how the scene is "supposed" to flow. Other groups prefer to have some major events and endgames in mind, then let the story build as players react to that event. I prefer this latter, "LARP-like" style as it gives players more freedom and lets players involve themselves with the story as desired.
3. This isn't so much my concern, but something I would like some clarification on. If someone plays a character who is inept at combat, are they likely to purposely perform poorly in-game? I've heard stories about this happening, but I always figured it was a rare thing. Thoughts?
I think this is an urban legend.
I hear non-RPers throw this out from time to time, but I've never seen any evidence of it in my many years RPing in MMOs.
What heavy RPers will typically do if their character is inept in a particular area -- let's say combat -- is to not level their character in that area, or never demonstrate faculty in that area ICly. L'yhta, for instance, has all the melee prowess of a wet noodle right now, so she's not going to have any levels in those classes and, being aware of her lack of ability, won't go out looking for trouble as a melee class. Some people who are OOCly playing non-combatants or characters without the Echo are going to just not do combat content ICly.
In the end, there's commitment to your character, and there's friendliness and care for fellow players. Since level is a proxy for ability, being low level in something your character's not good at but still playing that to the best of your ability OOCly is a fair way to represent someone who's not accomplished in that Discipline while not being a jerk to your fellow players.
4. How are spells like Raise and the like treated in RP? This is a pretty broad question I know, but if someone's character gets K.O.'d in like RP combat, I wouldn't want to do something that's immersion breaking and ruins it for someone.
In game, Raise and its ilk are described as restoring a person from unconsciousness. So, we can assume that when a character keels over, they're knocked out (and probably quite injured); Raise spells rouse them and heal many of their injuries. In IC PvE content, people pretty much have to assume that's how these work unless they want to spend hours on medical treatment RP instead of completing the content.
There's a few threads on these forums where people have discussed how healing magic can be handled in a world where medical treatments (alchemical and otherwise) exist in lore; Medicine in Eorzea is the most recent one. To sum up my stance on it, healing works however the players involved want it to work, with the proviso that neither side should godmode. That means the recipient of healing shouldn't refuse to allow magical healing to have any effect, but neither should the provider of healing assume that magical healing is a perfect solution without the consent of the person being healed.
(Optional) 5. What is your characters sleeping posture like?
L'yhta usually sleeps on her stomach, sprawled out. Sometimes -- when she's worried about something or sad -- she'll curl up in a ball on her side, with her tail tucked between her legs.

1. What's the best way to respond if someone meta-games?
There's a whole bunch of options here depending on how invested you are in the RP. For minor things -- knowing your name before being introduced, identifying your class when you've no visible indication of it, and the like -- my recommendation is to come up with an IC way they could have known that and roll with it. These sorts of minor infractions usually come from newer RPers and aren't really worth a great deal of concern. If you like, at the end of the scene, you might send the player a tell OOC thanking them for the RP and commenting that, as a bit of advice, they may want to avoid taking that sort of OOC knowledge IC.
For more serious infractions -- such as knowing private background information off your wiki page or your 100 Questions, for instance -- I think the best first step is to immediately send the player a tell OOC and, being friendly (naturally), ask them how they came across that information ICly. If you don't really mind them knowing it, you can offer some IC options by which they could have come across it, but make it clear that you'd prefer they find that information out ICly in the future. If it bothers you, or if you want to draw a line in the sand, you can offer a quick retcon of that part of the scene. That's one of the reasons why you have to react quickly; it's a lot easier for someone to say, "Okay, ignore that last thing I said," than to say, "Okay, ignore that whole last conversation we had."
If the player ends up being belligerent about it, or if the infraction is egregious (such as the player writing themselves into your background or knowing a deep secret of yours), just walk away. You always have the option of taking your ball and going home, so to speak. Respectfully tell the player OOCly why you're doing this, then inform them that you don't feel comfortable continuing the scene. If you and the other player share an IC LS or FC, you should also let the leadership know what happened; many such groups have policies about metagaming and may want to have a word with the player in question, especially if they're being awful about it.
2. For planned RP sessions, I've seen some of you said they're usually just simple "Let's go x and do y," but how exactly are more storyline RP scenes planned out?
This varies from person to person and from group to group. Some groups like to have a sequence of "closed" (i.e., not open to other RPers) scenes planned out, including how the scene is "supposed" to flow. Other groups prefer to have some major events and endgames in mind, then let the story build as players react to that event. I prefer this latter, "LARP-like" style as it gives players more freedom and lets players involve themselves with the story as desired.
3. This isn't so much my concern, but something I would like some clarification on. If someone plays a character who is inept at combat, are they likely to purposely perform poorly in-game? I've heard stories about this happening, but I always figured it was a rare thing. Thoughts?
I think this is an urban legend.

What heavy RPers will typically do if their character is inept in a particular area -- let's say combat -- is to not level their character in that area, or never demonstrate faculty in that area ICly. L'yhta, for instance, has all the melee prowess of a wet noodle right now, so she's not going to have any levels in those classes and, being aware of her lack of ability, won't go out looking for trouble as a melee class. Some people who are OOCly playing non-combatants or characters without the Echo are going to just not do combat content ICly.
In the end, there's commitment to your character, and there's friendliness and care for fellow players. Since level is a proxy for ability, being low level in something your character's not good at but still playing that to the best of your ability OOCly is a fair way to represent someone who's not accomplished in that Discipline while not being a jerk to your fellow players.

4. How are spells like Raise and the like treated in RP? This is a pretty broad question I know, but if someone's character gets K.O.'d in like RP combat, I wouldn't want to do something that's immersion breaking and ruins it for someone.

In game, Raise and its ilk are described as restoring a person from unconsciousness. So, we can assume that when a character keels over, they're knocked out (and probably quite injured); Raise spells rouse them and heal many of their injuries. In IC PvE content, people pretty much have to assume that's how these work unless they want to spend hours on medical treatment RP instead of completing the content.

There's a few threads on these forums where people have discussed how healing magic can be handled in a world where medical treatments (alchemical and otherwise) exist in lore; Medicine in Eorzea is the most recent one. To sum up my stance on it, healing works however the players involved want it to work, with the proviso that neither side should godmode. That means the recipient of healing shouldn't refuse to allow magical healing to have any effect, but neither should the provider of healing assume that magical healing is a perfect solution without the consent of the person being healed.
(Optional) 5. What is your characters sleeping posture like?
L'yhta usually sleeps on her stomach, sprawled out. Sometimes -- when she's worried about something or sad -- she'll curl up in a ball on her side, with her tail tucked between her legs.
The Freelance Wizard
Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))