Being a relative new user I asked a question and when it was solved I checked every tooltip for solved and did not dind a way to mark the thread as solved. Eventually I modified the subject line to say solved.

The tooltip on the check mark does not help. I haven't attempted it yet but can one mark more than one message as most helpful when more than one is involved with solving the problem?

Unfortunately not. Instead you can write a summary of the solution (i.e., what you did and link to the helpful posts) and mark that one as solved. Or you just mark the most helpful one.

By the way, if you mark a topic as solved, you can always revert it back to unsolved.