Ages since I climbed Cotopaxi, but IIRC I used Scarpa Mantas, I'm sure you'll be fine. I certainly wouldn't go to the trouble of buying plastic boots just for the trip.
I'd guess 3 or 4 out of group of 12 had plastic boots. No one lost any toes!
Leg or knee warmers are great for cycling, handy to keep the knees warm early in a ride and easy to take off when it warms up, but for walking they are practically useless, they just don't stay up.
I've one of the original Berghaus Paclite jackets from 6 or 7 years ago, and still regularly use it. Never had probs with it not being waterproof, and it's light weight (compared to full goretex) and breathes very well. I rate the paclite trousers too, excellent to shove in the pack just in case.
Munros, not Munroes of course. I'd agree that Knoydart's a super area and there's a few Munros there that you could knock off in 5/6 days. Also good is Letterewe, Fisherfield area with plenty of wild camping opportunties.
It's lightweight Inov-8's year round for me, except when I'd expect to be walking in significant snow. In winter I simply wear a thicker pair of socks.