1. The only thin I can find at all on Google related to T-mobile building is this post, suggesting they need a partner to start working on LTE in — get this — 2015: http://www.slashgear.com/t-mobile-lte-plans-suggest-spectrum...
2. It's certainly helpful; I don't believe this alone would doom a carrier, but without other benefits (Metro PCS, your example, has very cheap pre-paid plans and is really a different service than AT&T, Verizon, or T-mobile) this does hurt them. Just look at the millions of customers that switched to AT&T from 2007 to 2010 before Verizon got the iPhone.
3. I didn't say and, in fact, do not believe that "consolidation" will necessarily benefit customers. My only belief here is that that the government did here — keep a independent — is not useful because T-mobile is simply not competition. If T-mobile isn't actually competition in the future for AT&T or Verizon, how does it help for it to exist or be broken up among other companies?
(I do, however, believe it is possible (definitely not guaranteed, but possible!) that this move, in fact, could help consumers: AT&T currently has the generally accepted worst coverage, and so if they improved that by acquiring T-mobile, Verizon now would have both a slower network and a worse telephone network, and would need to improve to compete with AT&T's faster speeds and now-better coverage.)