Regarding establishing parents for Ewen and wife Isabella, Isabella's will be harder to confirm given her death before the start of official registration. You are likely to have to make some educated guesses on that!
Ewen McDonald, you would need to follow through with him to hopefully past the start of official registration where his death cert will include his parents's names.
Making a guess from what we have so far for him

there is this on IGI:
Ewen MACDONALD chr. 20 AUG 1818 in Laggan, Inverness
Parents Donald MACDONALD and Ann MACPHERSON
Monica
Added: The family in 1861, still at the same place:
Ewen McDonald 41, gamekeeper
Donald McDonald 19, school master
Ann McDonald 17
Isabella McDonald 15
Duncan McDonald 13
Address: Dalwhinniebeg