I have solved the problem.
I re-ran the split, this time detaching my wife from me (felt terribly guilty about doing this) instead of detaching her grandparents individually. This resulted in virtually no "islands" (and the ones it did leave turned out not to have been connected in the first place).

The islands resulting from my original split came from:
- sibs of my wife's parents, and their families
- sibs of my wife and their families
whom I had excluded the first time round. Their overall numbers are not large, but I'm astonished that they produced so many islands.

So my problem was a self-caused one, and I paid the price. Many thanks to everyone for their invaluable help.