In going over my notes and emails I find that I have been doing this since 1994. I think it is time to rethink.
I have postulated several siblings for Matthias. Sara has never fitted in quite right so I am suggest a different arrangement. Maybe Richard married around 1600 and produced two boys ( Matthias and James ) and then the wife died. He was desolate and didn't get remarried until about 1920. This wife was Sara and when she had the twins, Nicolas and Elizabeth, she hoped to ingratiate herself with his family with those family names. In the next few years, old Richard died leaving a young widow who took the 12 year old kids to Massachusetts along with Matthias and his family. There she eventually met and married John Gregory.
When she was about 20, Elizabeth married Walter Hoyt and subsequently died in childbirth. Then Walt married Matthias' sister-in-law Rhoda. Now, I can't prove a lot of this but I bet you can't prove any of it is incorrect.
Another possibility is that between wives Richard went to Ireland with his brother Oliver. That would explain the reports that Richard went to Ireland. He also may have had a family in Ireland. We're talking about a space of 15 or 20 years.