Dear Phil
When you say "John Mayhew (presumed sibling of Robert the younger) also has children James, Robert, Sarah and Martha." are you referring to the John Mayhew who married Elizabeth Adams at Westhall in 1770? If so, this John is probably christened at Hedingham in 1747, son of John Mayhew & Mary Saunders ("age at marriage 23" supports this). So he's not the one christened in Stradbroke in 1745; and his brother Robert is christened 5 May 1751 in Heveningham, again not Stradbroke.
Quite separately, I do see problems with the supposed 13 children of Robert Mayhew and Mathta Newman, who married 3 Aug 1742 Stradbroke. They are given as parents of Robert, chr. 25 Mar 1744 (or should that be 8 Jan 1745/6?), but of the others, three are given as having mother Mary (no maiden name); one of these is christened before the marriage of Robert and Martha, and the others on 26 Mar 1744 and 23 Jul 1745, which doesn't leave room for Robert to be their brother. So it looks as though there was a second Robert Mayhew, with wife Mary, having children in Stradbroke at the same time as Robert and Martha. And then none of the other nine children have a cited christening record - is there a definite link for any of them to Stradbroke?
Sorry to be so negative, but it would be nice to clear this up, in case Stradbroke is a wild goose chase!
Best wishes
Denis