The reason I asked whether you are sure Thomas Maxwell was born in Scotland, was because there is a decree for a John Maxwell [the name of Thomas & Ann's only son - who could have been named after a grandfather], on film:
MAXWELL, John 1765-1 Bra E d LDS film no. 0106388
which would be worth looking at, in case a son Thomas and other family are mentioned.

About your problem with finding Samuel Brew who was born in Lonan c.1762 there was a burial in 1827 in Lonan, and also the will previously mentioned, both from Brian Lawson's website:

BREW, Samuel 1827 Lon A 2 [film no.] 0106245

Wills & decrees are often a wonderful source for names of siblings, other relatives, places etc., as well as direct family.

Sue