I believe Jane Wyman was invited by the family. With her kids there, she would have been located with them. Delores probably wouldn't have received special treatment, but she would have had closer companionship than this other lady seemed to have.
I pretty much agree with your seating related viewing rotation comments.
If it was Jane surely Mike would have accompied her. Mike has some children did Maureen, does, Patti or Ronald have any or is this the only Reagan grandchildren.
Someone on the thread about his laying in state at the library before they brought him to DC mentioned that Jane Wyman was there at that time. Maybe she chose not to come to the interrment service so that there wouldn't be media chatter about her when it should have been about Reagan. It's also possible that she wasn't invited.