Invitation wording for parents who are still married but have different last names?

I think the way you have it is correct. The Mrs. before her name would indicate they are married.

Does she go by Mrs. or Ms.? I'd follow her preference on that one. If you put their names on the same line, rather than two different lines, that will indicate that they are married.

You have it correct. Simply place an "and" between their names, but keep them on the same line in the text.