You are right to be wary of some inclusions in the LDS index ... however, if the batch numbers begin with either a 'C' (for christening) or an 'M' (for marriage) then the original record will be found on SP.
Doubtful there were two marriages, the dates likely refer to the marriage banns read in the parish of the groom (in David's case, Ormiston) and the ceremony in the parish of the bride (in Helen's case, Pencaitland).
I wondered how you 'knew' that 'your' Alexander was born in 1782? The record on LDS shows a birth of someone of this name to father Robert in Peebles but the birth in 1778 of an Alexander to father Robert might also be worth investigating since it was in Liberton, the same place in which he was later to marry.
What were the professions of 'your' William and Alexander?