So this is known as a bug. All civ units already on the map when reclaiming will be friendly until a reinforcement arrives and siege begins.
The hostility status must be stored somewhere in memory. So it should be possible to change it with the aid of some memory changing tool. Something like Dwarf Therapist or dfhack or the like. Any advice on how to do that? I'd like to fix this in my active fortress.
The hostility status ... if I'm not mistaken, that's between the civilisations. Not the individuals.
So if he changes the elves to be friendlies he won't get any more sieges.
If that was the case, then those elves would've been hostile from the get-go. Their status
must be stored in memory somewhere.
By the way, I would be really interested in hearing about any memory-editing tools (or any other sort of tool) that would allow me to change relationships between civs in Fortress Mode. I've made a mistake with the orcs' [CAN_SPEAK] tag and now they insist on bringing me trash in wagons, instead of the bodies of their warriors. I've been seizing their goods and harassing their caravans, but they are still as friendly as orcs get.
Actually, I believe you could use Dwarf Companion to change the disposition of those elves. Or make them butcherable, if that's what you are into (Elf leather thong, yes). Won't work with civs though as far as I'm aware.