I agree with your theories. The thing I find disappointing is my inability to connect to my expected Mayflower ancestors on WikiTree Relationship Finder through marriage, as cousins or direct blood lineage, when it's so easy with Presidents and Magna Carta Sureties.
For example, of the 15 Magna Carta Sureties, 12 are my grandfathers, one is an uncle and two are cousins, but all are blood ancestors. Of my 36 Presidential relationships, all are cousins.
looking at one example: John Alden (Alden-63) is the third great grandfather of John Quincy Adams (Adams-12), as follows:
1. John Quincy is the son of John Adams,
2. John is the son of John Adams, 3. John is the son of Hannah (Bass) Adams,
4. Hannah is the daughter of Ruth (Alden) Bass, 5. Ruth is the daughter of John Alden.
So then, John Quincy Adams and Ron Campbell are both descendants of George Downing, 6th cousins, 6x removed. Again, the relationship is sons and daughters in direct lineage.
When I use Relationship Finder's 30 generation search between George Downing (Downing-244) and John Adams (Adams-63), I get "We could not find a blood relationship between John Alden and George Downing." The same response, "...could not find a blood relationship between..." me and John Adams is displayed by Relationship Finder.
Wouldn't the Mayflower passenger, John Alden and I and George Downing be at least cousins by marriage? So do you think that maybe that's what's happening; I'm related to a few Mayflower passengers by marriage, as cousins, but not blood, and WikiTree's Relationship Finder is tracking blood only relationships? That may suggest that my published family genealogy is still possibly accurate in it's ancestor (cousin) connections.