I'm a height-difference literalist! It is a matter of immense disappointment to me that there are quite a lot of descriptions of the historical Alan Breck, and he seems to have been reasonably tall. iirc, RLS's description of his clothes comes pretty much straight off a Wanted poster that also describes him as being 5'10", so the decision to make him a Short King was I think very deliberate.

I've never known quite why Alan speaks such braid Scots - Highlanders tending not to even now, especially if their first language is Gaelic - and I like your rationalisation for it.