True, but in my experience a possessee needs to have the object the spirit somewhere on their person for the entity to retain control of them.

Which in this particular case would mean carrying a 108 kilogram rock everywhere. Difficult and just a little bit hard to hide. Using a Pokeé Ball as a medium to reduce that weight likely wouldn't work, as that's designed to contain it's energy even further than it already is.

As far as accidentally releasing them goes, I haven't found any records of it happening. Apparently, it's difficult to do on purpose, never mind unintentionally.