If you have RM2 and RM: The Basics then you can just compare the them to determine the differences.
RMC is basically a reworked version of the old RM2 books. Lots better organization and some minor fixes. The two largest changes are the tactical system and spell types. But, RMC is essentially the same as the RM2 you have.
RM: The Basics is a cut-down boxed set version of RMSS, which was later repackaged into RMFRP. All three are basically the same thing.
RMX is part of the RMC product line. It is basically the core rules with a very few options put into place to make things slightly smoother overall. There is a thread at the top of the RM2/RMC/RMX forum that gives you a teaser showing the condensed combat system and the page which tells what options where used. The whole idea of RMX was to show that RM was not as complicated as some folks thought.....
More specifically, why go back to before RMSS and RMFRP?
Short version --- RMSS/FRP are, quite frankly, too complicated (in character creation) to be an initial buy-in product. it just cannot attract new players (or new gamers) as well as we need it to.
While there is some small growth with it, that growth is not enough to sustain ICE. Many old RM fans also refused to move to RMSS because they didn't like various aspects of it. (note: there is nothing wrong with RMSS/FRP as a system, it just doesn't enjoy the growth we need it to enjoy).
Whether RMC is working to reclaim that lost segment or not, I cannot actually say. However, sales of the RMC products have been phenomenal, with distributors wanting them like they were hotcakes.
RMX is meant to show that we can produce a FULL, playable game in a relatively few pages (88 pages, 10 of them being an adventure and other material, not rules). The print version sells for $10, what is essentially an impulse buy for many people.