Hi mama! Did you read the tongue tie sticky at the top of this forum? That helped me a lot when we were dealing with TT.
I can't really answer your first question...DD wasn't latching correctly, and she was certainly hopping mad, but I think it was from not being able to transfer milk instead of the latch itself.
We got DD's posterior tongue tie clipped when she was 3 weeks old. She actually needed it done a second time when she was 6 weeks old. She did learn how to nurse correctly after the procedures. It was kind of a learning curve, she got better day by day, and was nursing perfectly after a few weeks. There are exercises you can do with the baby after the procedure to help them learn how to use their tongue correctly. We were pretty lazy about it though.

It was tricky to find a doctor who could diagnose posterior TT. The first ENT we went to (recommended by our ped) said, "Well she's not tongue tied but I'll cut it if you want." That doesn't even make sense! We got another recommendation and the second doctor was wonderful. I'd just ask everyone you can think of in your area...your midwife, your ped, and an IBCLC. Actually maybe getting a recommendation from an IBCLC would be a good place to start. Make sure you ask for doctors who are knowledgeable about posterior tongue tie, otherwise you might get any old ENT. If worse comes to worse and you can't find anyone, I'd email Cathy Watson Genna (contact info in the sticky) and ask if she knows anyone near you.
Good luck!