I've decided to turn invaders back off on my 'Treasures in the desert' fort (fairly mineral-rich world, so of course i got coal and hematite and garnierite and kaolinite and shallow lava. But no trees on the surface, for what little that's worth).
It wasn't a challenge - put up some traps, turtle between the tenth of 'late <season>' and the tenth of 'early <season>', then collect prisoners and items. Except i was playing this with a pop_cap of ten, so no migrants past the first two waves. I had to disable all military training and the meeting hall just so the mountainous work backlog wouldn't bury them, but the gobbos keep bringing way more stuff than my midgets can clear out in the two months per season during which going outside isn't suicide.
With the watermark of 120 dead invaders and 25 prisoners (at sixteen adult dwarfs), i decided that this isn't fun. This isn't even dwarf fortress, this is Extreme Goblin Cleanup. Sorry invaders, off you go again.
But it was at least interesting to get to know you. I now know for certain that escaped ambushers _can_ show up in later ambushes. A bowgoblin squad was led by a human pikeman who upon entering the map dropped a glove and shoe - she was missing an arm and a leg, both of which were in fact sitting in my refuse stockpile, since she'd lost them in an earlier visit.