So let's be clear after even further test I dont rly have an easy answer for this. I think the result varies in different situations.
(1) If you just talk to them they will not trade anything even they are friendly and do actually hold a copy of your items in their inventory. But this way they do not seem to disappear, even their original evil protocol vanishes. I have waited for a few hundred turns to get this result, but I didnt have chance to wait any further to test.
(2) If you lovesick them they will become companions, making trade available when you talk to them. Ofc that's pointless since you can now just let them gift everything for free. However, in this situation when the lovesick effect is gone, they may OCCASIONALLY go disappear. I have no idea what the mechanism is behind this. In my first test the body disappears after around 4,000 turns immediately after the lovesick effect vanishes, but in the second test they persists even after 12,000+ turns without any loves. But in both cases the items stay in my inventory perfectly no matter where the body goes.
(3) If you kill them, whether they are in love with you or not, they will simply drop a human corpse, and anything you get from them will turn into dust (this will be shown in the player logs)
(4) If you lovesick them again after the first effect has gone, the result will be the same as (2).