My fiancé is being held in immigration jail right now. The only way for him to be released and not get deported is to get his felony case appealed. If we were to get married now while he is in jail (if that is possible) could he still be deported?

Yes, getting married to an american citizen does not make you an american citizen you have to file all the paper work to become a citizen

Agreed, the marriage will change nothing, he is not eligible to enter the US due to his criminal record. You have to decide if you want to wait for him to clear up his record (which the courts in his country may not do) or move to where he is.

Getting married now would cause you more problems. It would be seen as a scheme, which it sure sounds like. A marriage would not withhold any course of action against this guy.