Hi Rick
Looking at those two familysearch records, and in particular the Film Numbers.
The record saying he married Mary Musson is film No 1450477, which is the
Parish Register for Witham on the Hill
http://www.rootschat.com/links/01ksu/The record saying he married Sarah Musson is film No 508075 / 508095, which are the
Bishop's Transcript for Witham on the Hill
http://www.rootschat.com/links/01ksv/Therefore the transcripts are from two different records and will have been transcribed by two different people, and as such may be subject to human error.
The link I sent previously is to the images held at Lincolnshire Archives, which are itemised by them as being the Parish Record, ie the one Familysearch has filmed as 1450477 but transcribed as Mary.
I think your best course of action would be to email Lincolnshire Archives and ask if the image on their website for Witham on the Hill is from the Parish Register or the Bishop's Transcript. If they confirm that it is the Parish Register then the correct bride is Sarah.
As to Peter who died in 1795, I have a CD of Rutland Wills 1384-1858 and the only will for a Peter Roberts is the one you already have.