Sharon
Slowly we are chipping away at this - it's been challenging fun

A couple of thoughts here having immersed myself in Brodies in the last couple of days. I'll list them in no order of importance:
1. I do think that Lilly McBride is probably the Lilias showing as b. 1851 to Neil and Barbara. There is nothing for her in Scotland for 1861/71. It would be brilliant to find her in Canada for those years. I don't suppose you have been able to find her death cert. (?) to date. Her age on the censuses that you do have her for seem erratic. The 1891 one certainly shows her as born in 1851, as for the rest they are out by a few years but as with all recording of ages on censuses, -5yrs is not unusual in some cases.
2. I also don't suppose that Barbara's maiden name was included in her estate papers/ death cert. following her death in 1887 or you would have said. I am not familiar with Canadian death certs. Thinking of her children's death entries if you have them (?), do they not show mother's maiden name like the Scottish ones do?
3. One thought to bear in mind with that 1841 Census entry with Neil and a Mary in S Knapdale. I still don't think that Mary and Barbara are one and the same. It wouldn't surprise me if something you said earlier was not closer to the truth. It may be that the Mary Brodie is a sister to Neil (his mother was called that). It is possible, as you said, that first wife may have died by the time of the 1841 Census and that he married Barbara after this date. Hard to confirm either way unless we find parent details for the children showing in 1841.
4. There is only one other BRODIE family in South Knapdale in 1841, headed by an Alexander Brodie. His age would make him potentially a sibling to Neil, Dugald and John etc. Might be worthwhile following up to see whether he also left for Canada. Cannot see him from the 1841 Census onwards.
Regards.
Monica
ADDED: Re No.4 above. I think Alexander Brodie may have died pre 1851. This is the family's entry in 1841:
Alexr Brodie 42, agr. Lab
Christian Brodie 36
Mary Brodie 19
John Brodie 17
James Brodie 15
Duncan Brodie 13
Dugald Brodie 9
Alexr Brodie 7
Address: Baile Boidhead, S Knapdale
And in 1851, this is probably wife Christina with some of the children;
Christina McBride 55, mother in law
Duncan McBride 21, brother in law
John McLarty 26, agr. lab. b. Jura, Argyll
Mary McLarty 27
Alexander McLarty 1 Mo
Address: Baileboiach (cottage), S Knapdale - everyone apart from John McL showing as born in S Knapdale