I want a dying civilization,
Could you clarify what you mean by dying civilization?
I want kidnapping, but the goblin way of doing it threatens the limited age intended, so:
Can a civ be based around night trolls and spouse napping?
Can night trolls have four castes so I can have both male and female spouse nappers?
If the former questions are both true will they go out of spouse napping even if they might have the possibility of marrying within the civ?
Any kids abducted in worldgen are fully integrated into the civilization. I have no idea if they marry or not, and if they marry those outside their own race. If they do, they probably don't have babies.
You can't play as night troll per se, but there is nothing stopping you from making a creature called "night troll" with as many castes as you want and having it form civilizations. For kindapping mechanics in game, you'd make a reaction called "go out to kidnap RACE/victim/whatever" or whatever you want to call it, (optionally) have it transform whomever runs the interaction into an immobile no-eating/drinking/etc creature with no blood or organs called "troll out kidnapping" for X months, the reaction would produce an item called "kidnapped RACE/victim/whatever" or you could use DFHack to spawn it after the X months of transformation are done, then have another reaction called "convert victim" that would use DFHACK's spawnunit command to create a new creature of your civ.
I also want some Indiana Jones like adventure mode exploring of ruins, so:
Is there a way of making a species capable of becoming mummies while disallowing vampirism and necromancers without making a separate version of mummy making for the species?
I've seen some catacombs with traps, while most don't have that, is there a way to increase the chances of traps or maybe even guarantee it?
A creature can only become a mummy if they have CAN_SPEAK and do not have either the NOT_LIVING token or the SYN_CLASSes added when it becomes a werecreature/vampire/mummy. Vampires/werecreatures, on the other hand, require CAN_LEARN instead of CAN_SPEAK, along with everything else the mummy requires. You could, theoretically, create a creature with CAN_SPEAK but not CAN_LEARN. I don't know how the presence of INTELLIGENT would affect it, though. Keep in mind that INTELLIGENT and CAN_LEARN are needed for a creature to perform any labours, either in world-gen or regular play.
You can't affect sites themselves via modding.
I'm not that knowledgable on weapons/armour, so I can't be of much help there.