1. The English language version is pronounced Mak-aye (as in "aye, aye, sir!"). That's also the Gaelic pronunciation in some districts, though Mak-uh-ee (as near as I can get it) is the standard pronunciation.
2.I think that both spellings would have appeared and been used at pretty much the same time. I would just add that the chief of the name spells his name Mackay. There always seems to have been some difficulty with the spelling and pronunciation of this name in Scotland and in Ireland; Mackie, Mckee, Mckie and MacCoy are all versions of the name. The same is true in the Gaelic, where spellings MacAoidh, MacCaoidh, MacÁidh and MacÁi were found, though the first of these is now the accepted version.