Back on the 'is it mean to say it's wrong' thing. Sorry if this goes on a bit.
There are all sorts of reasons why those mis-spellings come about - I suppose it's possible that some of the original Mhairis and Vairis happened because people had a memory of the way in which the name was often used in the vocative when speaking Gaelic, probably within their own families, but because they'd lost the language didn't have enough Gaelic themselves to understand the grammatical significance of the shift in pronunciation & spelling.
Don't underestimate literary influences, either - big spikes in the incidence of Hamish, for instance, in the late 1820s which I bet is due to Walter Scott; and there are funny spikes in the occurrence of Mhairi which I bet also have something to do with literary fashions, some really awful pseudo-Celtic stuff among them.
There's hyper-correction too, of course - people knowing that Gaelic seems to have a lot of extra h's, and sticking them in for good measure. Stella MacCartney's husband Alasdhair is a good example of that.
But I'm not sure it's always that benign - we have to remember that all this takes place in the context of a 300 year old official campaign to stamp Gaelic out, prevent it from being spoken, disenfranchise people who did, and denigrate and remove their cultural heritage. In that context saying things like 'oh it doesn't matter how you spell/pronounce it you can choose whatever you like' smacks to me of the kind of dismissive - in fact downright hostile - attitude towards that value of that culture, language and heritage that allowed those anti-Gaelic policies to hold sway for so long. Fits easily into a colonialist view-point - 'we can take it and do what we like with it because we're basically superior to them'.
That's a message that's easily internalised, and absorbed by people who should be able to see themselves as owning that cultural heritage - particularly when they've had the linguistic tools that would enable them to resist it taken away from them - so many Scottish and other families do end up continuing 'wrong' spellings and usage, because they don't have the language, or the political awareness, or the impetus, to do things differently. Or because their granny spelt it that way and they loved them, which I don't discount.
I suppose why this spelling business exercises me to the degree it does is that it seems a relatively simple way to give a little respect to a culture and a language, and to try and understand it better - we're all richer for that, surely?