As the title says: addToRemainsCollector and removeFromRemainsCollector do not work on corpses in MP.
If you remove the unit before he dies, the corpse won't disappear. But using addToRemainsCollector on said corpse will not do anything: It will stay there for an unknown amount of time (I stopped after 15 minutes, corpse was still there).
isInRemainsCollector cursorObject when looking at the corpse will return true, if you had executed addToRemainsCollector on the corpse.
If you add the unit after he dies, the corpse will disappear.