Just because your character has "maxed all the classes," "gotten all the amazing gear," and learned "all those skills," doesn't necessarily mean you need to reflect that ICly. That's where you start falling into sketchy territory, and why you might be having a hard time trying to justify just how a character can be proficient in all these abilities.
If you do want your character to have skills they've learned from leveling different classes, consider limiting just how proficient they would be in those techniques. If your character is a Paladin, it wouldn't be a stretch to say that they're likely fairly good at hand-to-hand combat (something they'd have to know if they lost their weapon!), and learning the spear would not be terribly difficult. You might not bounce around like a Dragoon, impaling enemies with diving strikes, but anyone can reasonably thrust a spear and understand rudimentary techniques with it. It's not even something you'd have to enlist with the Lancer's Guild to learn.
As for magic, that one's a bit trickier because there's a lot of discipline involved in learning it, although we have seen evidence that some people can be naturally proficient at magic; Slyphie, for example, from the Conjurer quest line. But again, if you're trying to avoid godmode, perhaps you're just able to pull off minor feats that are unrefined but serve in a pinch, or you're passable at a certain aspect of magic. To use your example, Chester B. Arthur might be able to heal things, explode things, and stick things, but how well can he do any of them? Having enough magic potency to heal a tiny cut is a lot different from being able to mend bone or repair grievous injury. You might be able to make a tiny explosion that can serve as a distraction, but you may not be able to call down a meteor. And again, anyone can swing a sword or parry with a spear at range--but how well can they do it against someone who's devoted more time to it?
Regarding player age and how they're able to be experts at anything (or many things) in their 20s, again it all comes down to how much time and talent someone has. It depends on how their character has been written. A "master detective munitions expert with enough magical talent to destroy buildings" might not actually be as far-fetched as it sounds depending on how much training and time this character had devoted to those skills. If you're becoming a 'master' at things in the course of a few weeks or months, that's when it becomes eyebrow-raising.
If you do want your character to have skills they've learned from leveling different classes, consider limiting just how proficient they would be in those techniques. If your character is a Paladin, it wouldn't be a stretch to say that they're likely fairly good at hand-to-hand combat (something they'd have to know if they lost their weapon!), and learning the spear would not be terribly difficult. You might not bounce around like a Dragoon, impaling enemies with diving strikes, but anyone can reasonably thrust a spear and understand rudimentary techniques with it. It's not even something you'd have to enlist with the Lancer's Guild to learn.
As for magic, that one's a bit trickier because there's a lot of discipline involved in learning it, although we have seen evidence that some people can be naturally proficient at magic; Slyphie, for example, from the Conjurer quest line. But again, if you're trying to avoid godmode, perhaps you're just able to pull off minor feats that are unrefined but serve in a pinch, or you're passable at a certain aspect of magic. To use your example, Chester B. Arthur might be able to heal things, explode things, and stick things, but how well can he do any of them? Having enough magic potency to heal a tiny cut is a lot different from being able to mend bone or repair grievous injury. You might be able to make a tiny explosion that can serve as a distraction, but you may not be able to call down a meteor. And again, anyone can swing a sword or parry with a spear at range--but how well can they do it against someone who's devoted more time to it?
Regarding player age and how they're able to be experts at anything (or many things) in their 20s, again it all comes down to how much time and talent someone has. It depends on how their character has been written. A "master detective munitions expert with enough magical talent to destroy buildings" might not actually be as far-fetched as it sounds depending on how much training and time this character had devoted to those skills. If you're becoming a 'master' at things in the course of a few weeks or months, that's when it becomes eyebrow-raising.
