[skygazer]

yes, in most cases of full on possession or temporary walk in or what have you, it is a 'collective' that is in charge, so it's more than one spirit.

BTW, mentally ill folks don't realize they are disassociating, which says to me, you don't have to worry.

What you describe reminds me of other lives you may be living parallel to this one, or simultaneously. I have a sense that past, present and many possible futures are all happening at the same time, or really, that there is no time as you experience it here on Earth.
As for allowing entities to use your own bioenergetic field to experience themselves, it isn't something that I would agree to, and from what I understand, no truly ethical being would ever attempt it. Use caution.