"Your Samuel had two siblings, we think: Andrew and John"

If you look at MVR on-line at NEHGS you'll see that Samuel had a whole bunch of sibs.

"I do not have any descendants of your Samuel"
It's difficult to imagine that he had any.
He must have died bef 1761

1) A Samuel Nichols and Abigail Eliot married in Middleton 17 Nov 1742
2) There is a 18 Nov 1743 Samuel birth record in Middleton to a Samuel and Abigail Nichols
(It's difficult to consider that Abigail (Eliot) Nichols is not the mother of this 1743 Samuel
3) There is a death record in Middleton for an Abigail Nichols 27 May 1759 which
identifies her as "w Dea Samuel, a 39y"
4) There is a marriage record in both Middleton & Danvers, 26 May 1760, for a Samuel Nichols & Anna White.
The Danvers record reads: "Samuel, Dea., of Middleton, and Anna White"
5) There is a Samuel Nichols birth record in Middleton 14 Jul 1761 to a Samuel and Anna.
This couple are most likely the Samuel and Anna White (Mayflower descendant) who married 26 May 1760'

The family moved to Merrimac ,Nh

See
Mayflwr fam, Wm White pg 153 &
(Mayflower Whites pg154)