There is no "maximum term of probation" per se, although there can be a maximum term for specific charges, usually five years. At least one charge in Michigan carries potential lifetime probation. Violations can get your probation extended.

I'm not following your math. If the initial term was two years, and the extension was two years, two plus two = four.

What do you mean by "the actual definition of the original sentenced charge"? Odds are it has no relevance, as you were sentenced under the old law.

You are free to discuss early termination with your probation officer. I doubt that the P.O. will be interested in releasing you early if you haven't paid off your restitution, but it's not my call.