1)I always have the tweak pack installed, but romances still seem sluggish. Is there a way to play with that component and make it even faster?
Not that I know of.
2)Why not a Swashbuckler? As far as I know, there's no strength requirement for it, and the major bonuses are being able to specialize in bows and get a few extra points of AC.
Because Swashbuckler is geared towards melee fighting, but without any backstab
multiplier - they cannot specialise in bows, only melee weapons that a thief can use. They get +1 AC, THAC0 and Damage every 5 levels. Why would you send Imoen, with her 9 STR, into a melee fight? That's sheer madness. The only possible reason for giving Imoen the Swashbuckler kit is improved AC.
3)I specifically said I don't want NPCs to be at my level when they join. That encourages me to play solo as long as possible in order to get higher level NPCs, while I'd rather them just join at a fixed level. So if I pick up Aerie, she'll be at level 8 or whatever regardless of whether I pick her up in chapter 2 or chapter 6.
No you didn't. You said you wanted it fixed so that you get higher level NPCs. The game uses different creature files for all of the joinable NPCs, depending on your level. There is no mod that I know of that fixes all of these creature files to one specific level. What if you picked up Jaheira and Khalid at level 8 when you were only level 1? You are talking about TuTu/BGT here.