Hi Anne Marie,
Not sure who the Alexander MacCuish is - Mary McKiggen's husband Archibald MacCuish had a brother, an uncle and a grandfather named Alexander, so I expect the connection is there. Could be his brother, I suppose. I haven't quite gotten round to looking all the way through that side of the tree, so I can't say offhand.
The Glasgow connection is through Mary McKiggan's daughter, Mary Ann MacCuish. She ran off to Glasgow to get married in 1911 to Roderick MacNeil of Barra, but the belief is that they stayed with (distant) family, though its unclear which side.
Christina Campbell - not sure about her. She certainly seems to be related to Catherine Beaton/McKiggan being from Skye and all, not I'm not sure. Could been an illegitimate birth and she's named after her father, could be she was married before she married John McKiggan, or her mother remarried. Or its a recording error. To be honest, as she's not a blood relative (just married in), I'm not overly curious - though I might have a look next time I'm in Edinburgh!