What is minimally acceptable dress for the opera in Milan? I don't usually pack anything fancier than khakis and polo shirts.
You'll be fine. I've done La Scala in a suit and also in khakis and a polo shirt. There will be all levels of dress - even just a sport coat would be a great accent for your planned evening!
You mean, like, a sport coat without a shirt? :)
(It is Italy, after all!)
No, Italian men rarely take their shirt off in public. In fact, I don't think, other than at the beach, I have ever seen it.