3) This depends on how you want to deal with it. There are times when it could be appropriate and times where it seems like overkill in a really big way. My own ruling for it has been, when you're significantly outnumbered then you count as larger, when you're not, then you do not.
4) Gangs that slam into each other don't need a seize by force roll to determine the results. The harm and armor and size tell us that, you just narrate the translation into the scene. If character A has a gang and they storm an enemy position, but instead of pushing his men forward with seize by force and using the gang like a weapon, the character is doing something else, like taking an opportunity to locate the enemy leader (ex: read a sitch), then you say sure! If they miss the read a sitch then your hard more should or at least could involve the character's vulnerability by targeting the non-directed gang conflict. But either way, I'd have the gangs exchange harm with each other as established. It certainly benefits of the gang leader to use seize by force to directly command his gang into battle, if for nothing else then to guarantee fictional objectives through the choices under seize by force. I would not ask him to roll as if he was doing both, he should decide which. It can be very beneficial to locate an enemy leader while two gangs war, because if you drop the leader fighting just you and some of his guys right in the middle of the fight, you can force the other gang to route that much faster.
5) This topic has many ways to play, the short answer, is do what feels right for you and your crew.
The long answer based on my own play is as follows: When you change into a new type, it represents a significant change to WHO this character is and how they define themselves. Say you have a gunlugger that has is fueled by hate, finally settling down to rule over a hardhold... This represents a significant change in the story of the character, it should matter. It should not be, Im ready for multiclassing now... It should be well maybe he's got a family now, he's settled down with Silver and had a kid. He isn't so ready to die anymore, hes got more to live for now. Whatever the case may be...
You and the player both look over the moves he has already, and whatever moves don't apply immediately to the new life he's creating for himself, drop. Maybe he isn't insano like drano anymore, maybe he's not brutal anymore, maybe he's not so violent and destructive and NOT TO BE FUCKED WITH just doesnt apply now cause he has things in his life he doesn't want to lose, so he isnt willing to put himself out there to die so easily. Think Fight or Flight, if hes got nothing to lose, he'll fight a lot harder then someone that might be distracted by the fear of losing someone dear to him.
Say the gunlugger going hardholder had Fuck this shit and Easy to Trust. Well as he's settled down in a hardhold he's probably not willing to bounce at the first sign of trouble anymore so unless he's still that kind of guy... probably not, but maybe whatever made him easy to trust before still applies now. So keep that one.
I always let them choose at least two moves they have and want to keep if they have them automatically selected as moves from another playbook on their new blank character playbook. Other moves are a bonus if they get to keep them. Keep them only if they make sense to the new lifestyle and narrative logic that determined the class change to begin with.
6) When a character is stunned by a weapon, he is acting underfire to do anything. If its charge the bastards that stunned him, he rolls a seize by force, then rolls act under fire. Seize by force determines the non stunned result, act underfire describes doing it while stunned. I'd say they'll stay stunned until either they get the fuck down, they get out of that area, or enough logical time has passed that they get the chance to clear their head. This one is really a case-by-case thing.
7) A move does not represent a turn. You do not take turns fighting, the move represents the Scene, or part of a scene. If your gunlugger seizes THAT BUILDING FULL OF ENEMIES by force, one roll explains using a bunch of weapons maybe, tossing grenades firing guns, getting shot, etc. Take definite hold, he's the guy with the building now. The other people dont have the building, they might be inside fighting for it back, or they might be routed and forced back to another position. Do not use seize by force to do a play by play fight.
In apoc world, Everyone has Gas and everyone has Bullets. At least, that's the standard. I handle those things a bit differently now and then, but still. Do as the fiction demands, if it doesnt make sense, dont do it that way. You have a priority to make the game feel real while still being a fan of the characters,