Here's my two cents:

1) Is demonic possession real?
No, demons aren't real.

2) I have heard that hypnotism causes demonic possession. How/why is this?
Hypnotizing somebody can cause them to do things typically associated with demonic possession.

3) Other reasons for possession: how/why?
If someone thinks that they may be possessed, they can manifest the "symptoms" of "demonic possession." It's just a placebo effect.

4) Do nonbelievers of do get demonic possession? Why?
Demonic possession is a lie, ergo no.

5) Do you know anyone who gets demonic possession? What did s/he get?
I've known bipolar people, schizophrenics, epileptics, etc. If that is what you are willing to call "demonic possession," then yes, I do. As mentioned earlier, I don't believe in it, so no.

6) Is there an explanation without god/devil on demonic possessions? What is it?
Placebo effect.