I saw it in cinema this Sunday, and I noticed something I had not paid so much heed to before that makes me certain now that yes, they were married.When Jill is ransacking the drawers, at one point she takes out a small white bouquet (lillies of the valley? - apparently a favourite for weddings) and lingers over it for a very short moment. I realised that that must be her wedding bouquet which Brett kept as a memento - why else would it be there?And I don't mean that question just in terms of the world of the film; I think it's a visual clue to the viewers: here's a bittersweet memory.
Good observation, I also remember just recently seeing a close up screencap somewhere of Jill's hand and her wedding ring someplace.
I bet someone faxed a copy of the marriage certificate to Lionel Stander's waystation