I suspect the answer(or at least part of it!) is
I am only two letters, in troth,
But expanded, I can be more than both.
ST is two letters and is an abbreviation for words with more than 2 letters.
I might be here on behalf of a holy one,
Or one that's full of shops and houses.
Saint and street.
I might be the shell of a noun or a brand,
Or of the noun's verb in present or past.
Seat/sit/sat, all three words enclosed by ST (thanks Riley!) And Seat apparently is a brand, which I have never heard of (thanks Phylyp!)
Neighbours and yet in estrangement,
Find me and my rearrangement.
S and T are next to each other in the alphabet, and they appear in the words eSTrangement (thanks Phylyp!)