If you have evidence as to Grace being Jonathan's wife, then the best route to go would be to create the profile and leave the surname as Unknown. This can be marked for the Unknowns Project for someone to work on, and hopefully her surname can eventually be discovered. After creating the profile, you can add her as a marriage to Jonathan and shift his children from Thayer to her, and disconnect Thayer from Jonathan. However, I would wait to hear from the PMs first, because it is always polite. If they do not reply within, say, a month or so (by looking, their last contribution was in April), then I think as long as you provide sufficient evidence and you've let them know, you can go ahead and do it.