Right now, I don't think there's a problem with the new drummer.
I think the main problem right now is legalities with MP - royalties, if he is going to be bought out, if he wants the guys to use Dream Theater as a name, Official bootlegs (keep in mind, the Official Boots are a big part of DT's income and MP has all of the DT recordings). Again, I'm only speculating but MP could even sue for the band's name because he (his father) came up with the name.
Even if the band "left MP with the doors open" to "come back", there's a whole bunch of money that's going to be made when DT releases the album, tours, sells merch, and the new batch of official bootlegs come out (especially the last 3) so this has to be sorted out in a way that neither DT (band) or MP get screwed. Each party will argue what's (how much) corresponds to what and negotiate.