yes, but it doesn't sound good. "הוא יושב מאחורה" means "he sits backwards" as well and should be used
Yes, "he sits in back" is correct colloquial English. To include the word "the" is slightly higher register English, but to talk about sitting "in back" is perfectly correct.
Actually that's perfectly acceptable, conventional English. The definite article can be present or absent depending on how one construes the situation.
I don't know about America, but in Britain no-one would every say "He sits in back." It just sounds wrong.
That would imply that domeone else is in front, while "in the back" does not necessarily.