Question for kilt historians and anyone else who wants to throw in their 2 cents

[David Thorpe]

That's the sticky point. Since there is no Chief of MacDonald of Lochalsh, that would mean that I need to get the authority from the Clan Society...wonder if that would mean something like Clan Donald USA? That is a Clan Society isn't it?

I would submit that since your family was absorbed into the Sleat clan so long ago, that you are Clan MacDonald of Sleat. End of story with an easy solution with regards to what tartan you should wear.
Clan tartans are meant to be recognized, and identify the wearer. A new tartan would not do that.
Just my opinion, of course, to be taken for whatever it's worth.