To answer your second question;
IIRC The Penicuik artist (who drew the Jacobite army in Edinburgh prior to its march south) depicted many of the figures in breeches. Only a few had kilts.
I used to have a collection of these drawings in a book with a good narrative about them but I think I gave it so a friend or sold it, so I'm afraid I can't check. Some of them are shown online if you have a good google - the NW Gaidhlig Society has several in their site. The book is called Witness to Rebellion.
Now there is some debate about the drawings, but in terms of clothing at least they're probably quite accurate. It would seem that given half a chance the kilted highlander seems to have jumped into breeches or perhaps trews and kept his plaid for a blanket on the march.
Another point to bear in mind about the plaid was that it may well have been thrown off prior to a charge, with the highlander fighting in his sark. There's a fair amount of wool in a plaid and when it gets wet, it gets heavy.
Anyway....a mix or breeches and kilts for the highlanders would not be inappropriate.