Sorry that I didn't see your message before now! Most of the discussions moved naturally to the forum, as the recent postings appear on the homepage of the website.
As for your questions..
All research I've come across suggests that M(a)cCrohan/McCrohon/McCrehin etc were anglicisations of an Irish name called MacCriomhthainn, which originated in South-West Ireland. The "Mc" and "Mac" do not signify Scottish or Irish, and both forms are found in both countries.
It would appear that the various forms of the name occurred as our ancestors' names were recorded in Population surveys (census) or immigration controls in North America or Australia. The officials whose job it was to record their names, would attempt an approximation to the best of his ability, and this created a number of different variations. Remember that our ancestors at that time spoke Irish (Gaeilge), and it is likely that they were not literate, and could not propose a spelling of their name in English.
John McCrohan (Kerry) suggests that the version of our name which is closest to the pronunciation in the original Irish is "MacCrehin" and I'm inclined to agree with him.
These variations do not facilitate the work of the researcher, but it is a reality that we have to contend with!
Note that there are many stroies of McCrohans/McCrohons coming from Scotland on the website, but it is highly probable that their ancestors came from Kerry in Ireland. We even find records of American McCrohans who travelled from Ireland to Australia, and then on to America.
All the best