Well you certainly seem to have a pragmatic outlook about the claims of your husband's family.
The thing is: it DOES upset Scots when people make spurious claims... and just because something is 'absoutely gospel' by one family's history, don't make it so!
The Mc with the raised small 'c' was just a way of writing 'mac' - it was the fashion in those days - it has absolutely NO basis in 'It makes me more Scots' or 'It absolutely HAS to be written that way'
I hope you have some success in tracing his family back, but the thing is: even if he could claim 'Irish-American' status - many Scots actually settled in Ireland (look up Plantation) for one, two, three or more generations before emigrating to the USA and other places like Canada and Australia. Maybe that's where his family's 'folk memory' originates - but who knows!