better ask over at the wiring forum, but a counter appliance can be plugged into the 20 GFCI convience outlets serving the counters. If "building in" the micro which it sounds like you are then better get a dedicated line. If your micro is powerful : 1200 + watts then better get a dedicated line.

You cannot wire your can lights into the kitchen counter outlets. This would be dumb anyway. It's night your micro trips the circuit, and now you're standing in the dark. If wired properly you should have to worry about nuisance tripping and when it does trip you'll know.

If you really need the info as a security blanket then I'd suggest getting a GFCI outlet with trip lite instead of the GFCI breaker. They're cheaper to boot. If you don't like that look, get a LED nite lite to plug into a receptacle.

I would say that if this is a small (700W or so) MW then it's OK to plug it into outlet on same circuit as countertop outlets. If it's bigger or really builtin like a OTR MW/fan combo or one of those new MW drawers then I'd go dedicated.

If you only have a few cans, I think your idea of using the fridge circuit is OK, but don't know what inspector would say...

You should definitely ask the wiring forum, I also added a link that I find useful for review. It mentions specifically that your fridge configuration is not allowed:

Here is a link that might be useful: Electrical wiring guide