"Tim is going to come back at 5" and "Tim is goint to be back at 5". Would you please to explain me the difference between these two sentences? The second isn't accepted. Or in the second variant Tim is more confident and has already decided (strictly determined to) be back at 5 as contrasted with the first variant where he just planning?
"Tim is going to come back at 5." = "Tim will return at 5." Vs. "Tim is going to be back at 5." = "Tim will be here again at 5." There's not a lot of difference between the two. I might choose the latter, though, if I really have no idea when Tim might return, but I know he has to be present for a mandatory meeting at 5.
I am not sure we are going to accept "will", ever. The use of собираться here means that Tim's plans are to get back at 5, which does not mean that's guaranteed to happen.
Of course, "future" tense in human languages is always an estimate since we are seldom 100 % sure until the thing actually happens.
Maybe in English "will return" and "going to return" sometimes can be interchangeable, but in Russian "вернётся" and "собирается вернуться" are different. The latter is on the "planning to return" side of "going to return". It doesn't really say anything about what will happen, only about Tim's intent.