UT, good points all around. I agree that this "social entrepreneurship" stuff is sort of a catch-all category for anyone who simply wants to do something socially conscious with their career, which makes it tough for us all to talk to one another - someone who works for an educational foundation or philanthropy doesn't really have that much in common with a person heading up an NGO or social business entrepreneur. Nevertheless, I think it's good for us all to share our ideas about how to make these careers work.
Have you heard of the L3C corporation model? It's a really awesome new business format created by the state of Vermont to address a lot of the points you raised. Link here:
https://en.wikipedia.org/wiki/L3CAlso, most of us are taking out big loans for our B-school education. Frankly, I think a lot of the hysteria expressed here over being able to pay back your loans AND have a decent lifestyle are greatly exaggerated - no, as a non-profit worker out of B-school, you won't be living like an I-banker, but isn't that a good thing?
Salaries in this field are not as competitive, and they sadly never will be. Part of your "compensation" is the knowledge that you're doing something more important and valuable for the world than just turning a buck.
I applied to Texas too. The flexibility of their curriculum and the SE concentration appealed to me too... but ultimately I think I'm going to a school that seems to have a lot more resources in this field (Fuqua) and seems to take it a little more seriously. I wouldn't worry too much about shutting yourself out of different careers... anything can get old after a while, and this career path is no different. With an MBA from a great school, I think you'll do just fine.