Yes, you can't do that for abandoned creatures in the editor, but you can for the others... then you can set it using SetObjectOwner(). As for the error after the battle, I don't know, I just checked it myself, and it worked fine...

Also, I have a rather strange question... why doesn't this work?
function HelpSin ()
MessageBox("/Maps/SingleMissions/rework (4)/sin.txt");
RemoveObject("sin");
Trigger(REGION_ENTER_AND_STOP_TRIGGER , "sinRegion", nil);
end;
Trigger(REGION_ENTER_AND_STOP_TRIGGER , "sinRegion", "HelpSin");I have a lot of similar functions, but this one specifically doesn't work...
Added 8 minutes later
Oh, the second problem was solved simply by the fact that the region was surrounded around the unit on the map, and since this area is already considered as... an aggro zone for this unit, and the hero doesn't actually step into it... in general, I just stretched the region, and everything is okay.