"In two minutes" means in approximately two minutes, possibly more, possibly less, but not that much more or less. "After two minutes" means in at least two minutes, possibly a bit more.
They are entirely different in meaning, though there will be some contexts in which either is possible.
"To" with a verb of motion ("Come back") expresses the destination or target. "I'll be coming back to you" is a simple statement of intention, with no necessary implications.
"For" is most often a benefactive, so a possible meaning of "I'll be coming back for you" would be "I'll be coming back for your benefit" (though exactly what that benefit would be is not expressed). It does not even say that I'll come back to you: for example, if you have been accused of some crime, but I am guilty of it, I might mean that I'm coming back to give myself up, and get you exonerated - you might not even see me.
But actually, that interpretation is less likely than another meaning of "for" which is "for the purpose of". Again the precise interpretation is context dependent: if the object is an event, it will usually mean to participate in the event (I'm coming back for lunch/for Christmas/for the lecture). But with a personal pronoun, it will be interpreted as "to get you, or take you somewhere".
So
I'm coming back to you. = I'm coming back, and I will come to where you are.
I'm coming back for you = (most likely) I'm coming back in order to get you and take you somewhere.
Best Answer
The meaning is roughly the same for both versions: you should arrive before 10, however there is a difference in when the two versions would be used.
before tends to be used when the action has no lasting effect. For example, if you have to pick up a document and then leave, but the person who has the document will be busy from 10am, then any time before 10am is acceptable, somebody would probably say:
by tends to be used when the action has some lasting consequence that will still be in effect at the indicated time. For example, If you have a meeting with somebody at 10am, you need to arrive before 10am, and you still need to be there at 10am, somebody would probably say:
Another example for by would be if you have to submit a report before a certain date, because once a report is submitted it stays submitted.
Parents generally tell their children to be home by 10pm (ie they stay in once they have arrived) rather than before 10pm.