Did Michael and Kay miss Connie and Carlo's wedding?

Maybe they arrived late and had to sit in the back. That part of the story was off screen so we have to make up our own story. I based my guess on attending a huge Italian American wedding once. The church was packed. Some did end up skipping the service and went straight to the recption.

Who cares?

Michael was in uniform, so I guess it was a logistics thing getting home on leave. Could have been a delayed military flight or even any travel hold up that can happen when travelling long distances.

it may also have been a symbolic way of showing how distant Michael was from the family at the beginning - and the feeling seemed to be mutual - Michael and Kay didn't exactly come across as apologetic over the fact that they'd missed the ceremony, and Connie (and presumably Vito) didn't feel it was worth pushing the wedding date back so that her brother would definitely be able to attend.
He's greeted warmly by Tom and Fredo, but there's only a very brief unspoken acknowledgement with Sonny. Not sure if we see him reuniting with Connie, but there's a deleted scene in which they go to pay their last respects to Genco that night, and Vito, seemingly speaking to Michael for the first time since he's been home, can only make some snarky comment about his medals. (sadly they forgot to delete Sonny's "pop had Genco, look what i got" line much later on in the movie, which doesn't make much sense without this scene!).
And for Michael's part, you get the impression he doesn't want to be there, is only putting in an appearance at the reception out of obligation and feels uneasy about having to introduce certain people and explain certain things to Kay.
You could also say it's symbolic that Michael's still wearing his military uniform, as it makes him stand out from the rest of the family, especially his brothers when the photos are taken.