I have interpolated the siblings from the fact that they traveled together(or at least they ended up the same place) to Mass and then to Hartford CN. Elizabeth's husband is praised in Matthias' will. Sarah also went the same places. Nicholas came to America separately but ended in CN with Matthias. Their properties were quite close together.
Nicolas and Elizabeth have the same birthday and both came to CN. I have seen a references to Elizabeth being born in Wilts.
I have Oliver St. John; John born 1478; John born 1502;Nicholas born 1535 and Richard (third son)1562. Listed somewhere as unmarried in 1592. (I am not too good at keep my references straight.)I don't know anything a bout a Russel.
In 1999 I bicycled to Basing and the superintendent there said that "given the reputation of the St. Johns in Swindon we might be cousins."
Also,in Purley Magna a local woman said that Nicolas and Elizabeth were intimates of Henry VIII. But she didn't know anything about Richard except what it said in the church.
I don't deny that a lot of what I surmise is conjecture but I think it fits together pretty well.
I gather that you haven'tbeen able to connect the DNA of Matthias' descendents with those in England or South Africa.
I keep hoping that the Mormons will find a parish register that lists Matthias.
I agree that we will probably never solve the whole mysrery.