There's no way the DOJ would let Sprint buy T-Mobile anyway; one of their arguments against AT&T buying T-Mobile is they believe T-Mobile should be kept alive on its own and not be acquired by another telecom. Period. This stance effectively kills off any other rationale for why Sprint may be better suited for buying T-Mobile since much of those arguments tend to focus on Sprint being smaller than AT&T.
Furthermore, Sprint, like T-Mobile, is also slipping, although at a less frightening pace. Regardless of whether T-Mobile goes away because it gets swallowed by AT&T or if it simply goes kaput, Sprint's screwed anyway. Spending all of their efforts trying to stop the merger is one of the dumbest things they could be doing IMO; they should be working on improving their network and their service. Sure, they rolled their "4G" service first, but even T-Mobile and AT&T's fake "4G" is (theoretically) faster. Both Sprint and T-Mobile are fumbling around with their "4G" plans with T-Mobile scraping by with what they have and Sprint being unable to make up their mind and develop a coherent roadmap. Sprint's problems are ultimately their own fault, and they're the ones putting themselves in danger.
As far as UMA goes, last I heard, it still wasn't supported on their Android phones; it seems as though T-Mobile isn't really continuing very strong with that technology.