You have some great ideas here. However I don't like the meteor idea, such a meteor would leave no survivors if a meteor of that size was to hit Earth it would have the strength of three atomic bombs. I am strongly against ideas that end the game 'just cause'. I am also against the construction destroying ideas such as Earthquakes. Dwarf Fortress is about building megaprojects, not about having them collapse after a certain amount of time.
Also the infiltrator idea amuses me, but there should be a few more options. The leader could talk to them, perhaps have them leave without taking anything or at the least they will lower their demands. Or you could siege them by locking their door and after a few months they will either give up, attack each other, or kill your dwarves then go down swinging. I'm not such a fan of the 'homebase' idea, or that the leader has certain skills, it makes it to predictable. However, the dwarves could go into a dining room, zoo, or any other area. Their job will say 'No Job' when all living members get there, and there is a certain number of civilians there (three? Four?) they reveal they are infiltrators, lock all the exits, trap all civilians in cages. They will make their demands to the player. Then their demands will appear, you will be given four options (Give them what they want, Send in your leader, Send in Military, Ignore) if you chose to give them what they want you will be given a number of other choices to stab them in the back. Literally. If you wait to long to give them what they want, either by choice or by accident. They will either start pulling levers (if any) messing up furniture and engravings, or rarely killing off a dwarf.
Perhaps one or more of the infiltrators is very happy to be at the fort. He could rat out the other infiltrators and those infiltrators will automatically be targeted for hammerings and execution. Perhaps the leader himself could be so happy at his gold studded room that he decides to stay, at that point the rest of the group settles in your fort.