First off, remember this...you're pouch can do whatever it chooses, whenever it chooses! 
Follow that rule and all goes well! 
1. What causes that tight, painful feeling in your chest besides overeating or not chewing.
Your yoghurt could have been 1 degree off (either too warm or too cold) for your pouches rules THAT day. That's the lesson I'm learning too. My pouch HATES eggs and toast (even itty bitty samples) and doesn't allow home-made stir-fry for whatever reason. It's the pouches rules...we don't have to understand them...just obey!
2. With that said, will the foods my pouch can handle change over time?
From my understanding...your pouch preferences can definitely change over time! That's the good news!!
3. I may be eating out for the first time this evening with hubby and friends - what is it like to eat out post-gbs?? I am a little nervous - not so much about the eating part but more about what the people we're eating with will think. How did you guys handle it?
You're braver than I am!! I do not try anything new until I've tried it at home!! I'd hate to get that "Oh, please just vomit and get this over with dry-heaving" feeling. 
I know I'm not much help, but I am sarcastic...and that's a good thing, right?? 