Everything in your post is consistent with my T-Mobile roaming experience, except that I could not connect to Band 12. I'll be back in that neighborhood early next week, so we'll see if that's changed at all.
This doesn't kill the merger talk. This is just 4G. Sprint is going to need a lot of money if they want their 2.5 Ghz spectrum to really work.
There are still too many outdoor spots were 2.5 Ghz doesn't reach in major markets today. I don't think I have seen a new Sprint macro tower go up in my area in ages now. Hopefully after the merger, the EBS/BRS band is put to good use.