
I'll answer the more general questions because I'm not a warrior.
Defiance: If the fight has no adds, don't bother with Defiance as an off-tank. Should things go pear-shaped and death causes you to be the main tank suddenly, go ahead and pop it. If you're going to be holding adds for any meaningful amount of time, go ahead and pop it, but drop it once they're dead. There's no reason to reduce your damage and increase your HP if you're not taking damage to begin with.
Slashing Debuff: There's a lot of debate as to whether the WAR or the NIN should be using the slashing debuff. WAR will have the chance to apply it sooner in a fight due to NIN opening (or should be, elitism plug goes here) with their DoTs, but as long as someone does it it doesn't matter. If there's no NIN because you're doing Trials or something, then yes, debuff that target.
Goad: Tricky. If there are other physical DPS in the ground and you're only the OT, it's likelier than they'll need it more than you. If the party comp is just PLD WAR NIN BLM BLM SMN SCH WHM then feel free to ask for it.
Berserk: As an Off-Tank, treat Berserk like a DPS cooldown. Hit it as often as possible, try to not let it get clipped by invulnerable phases (Titan jumps, transitions, etc).
Defensive Cooldowns: I use mine sooner rather than later due to the effective-HP thing. If you have exactly 10,000 hit points, popping a 20% reduction at full health gives you 12k hp to tool around with. Hitting that same buff with 5,000 health "only" gives you 6k effective HP.
Convalescence: It's off-GCD, and it's just icing on top of Defiance's bonus. Use it in conjunction with your "oh shit I'm dying" reflexes.
Fracture: Controversial on paper. It'll devour TP, it's only ever a DPS increase if you get the full duration on the DoT and even then, the increase is almost inconsequential (last numbers I looked at showed an increase of 7 DPS over an entire fight). Some swear by it, some never bother, everyone's got their reasons.
Overpower: Overpower is too damn good. It outscales Flash by miles due to Berserk buffing it, and with an adequate weapon you might not even need to shuffle in Flash. Overpower > Overpower > Begin weaving your combo amongst targets.
Threat: When maintanking, don't put too much stock into your threat lead (doubly true if you're tanking a dungeon or am the sole tank). Yes, you want to be the top threat person, but as long as you're ahead it doesn't matter by how much. Feel free to try and maximize your damage while not dropping hate - WAR is a tank, yes, but they can put out pretty outrageous damage compared to their paladin counterparts.
Defiance: If the fight has no adds, don't bother with Defiance as an off-tank. Should things go pear-shaped and death causes you to be the main tank suddenly, go ahead and pop it. If you're going to be holding adds for any meaningful amount of time, go ahead and pop it, but drop it once they're dead. There's no reason to reduce your damage and increase your HP if you're not taking damage to begin with.
Slashing Debuff: There's a lot of debate as to whether the WAR or the NIN should be using the slashing debuff. WAR will have the chance to apply it sooner in a fight due to NIN opening (or should be, elitism plug goes here) with their DoTs, but as long as someone does it it doesn't matter. If there's no NIN because you're doing Trials or something, then yes, debuff that target.
Goad: Tricky. If there are other physical DPS in the ground and you're only the OT, it's likelier than they'll need it more than you. If the party comp is just PLD WAR NIN BLM BLM SMN SCH WHM then feel free to ask for it.
Berserk: As an Off-Tank, treat Berserk like a DPS cooldown. Hit it as often as possible, try to not let it get clipped by invulnerable phases (Titan jumps, transitions, etc).
Defensive Cooldowns: I use mine sooner rather than later due to the effective-HP thing. If you have exactly 10,000 hit points, popping a 20% reduction at full health gives you 12k hp to tool around with. Hitting that same buff with 5,000 health "only" gives you 6k effective HP.
Convalescence: It's off-GCD, and it's just icing on top of Defiance's bonus. Use it in conjunction with your "oh shit I'm dying" reflexes.
Fracture: Controversial on paper. It'll devour TP, it's only ever a DPS increase if you get the full duration on the DoT and even then, the increase is almost inconsequential (last numbers I looked at showed an increase of 7 DPS over an entire fight). Some swear by it, some never bother, everyone's got their reasons.
Overpower: Overpower is too damn good. It outscales Flash by miles due to Berserk buffing it, and with an adequate weapon you might not even need to shuffle in Flash. Overpower > Overpower > Begin weaving your combo amongst targets.
Threat: When maintanking, don't put too much stock into your threat lead (doubly true if you're tanking a dungeon or am the sole tank). Yes, you want to be the top threat person, but as long as you're ahead it doesn't matter by how much. Feel free to try and maximize your damage while not dropping hate - WAR is a tank, yes, but they can put out pretty outrageous damage compared to their paladin counterparts.