Yes. The possibility most certainly exists (although pregnancy could occur, not directly from the condom coming off, but from the semen that could be left inside the vagina after the condom came off).
the vagina acts as a vacum sucking the condom up slightly and will then to come off the penis if it has softened slightly before coming out of vagina
Yes it is possible.
Yes. The condom is supposed to go on an erect penis. If the penis is flaccid, the condom can slip off.
the condom- no. the sperm in the condom- yes
Then you defeat the purpose and effectiveness of the condom.
No, providing the semen stayed inside the condom.
Until your penis becomes limp and then the condom will slide off.
you should not need to change condom unless your condom comes off or you change sexual partner... You can use the same condom with someone else in the same session but it is best to replace the condom if it comes off.
Yes, he can as the condom can rolled off in much the same manner as it was rolled on.