Cotton long johns suck really bad. Ever notice how they never keep you warm on a cold day?

Winchester and Remington make the polypropoline (sp) underwear and you can get them at Walmart for a decent price. Just don't dry them in a dryer on high heat cause they could melt. Air dry them. I know that it may be a bit colder in Montana than here in Colorado but I rarely wear long johns in September even though I do bring them. It's a bit chilly in the morning but by the time the sun has been up for an hour I have to shed some clothes any way and usually end up in just a long sleve camo T-shirt and a pair of camo jeans.
Your guide should have all the tools needed to quarter and / or debone an elk. But, I would bring a bone saw any way. Don't trust any one else to remember to have it when it's needed. Gerber makes a very light weight one that the blades slides inside the handle. Again, Walmart has them for ten bucks. I use this one and it works great.
Pack boots suck unless you are going to be tromping around in foot deep snow every day which is very doubtful. Leave them at home. Your outfitter is right about them not working with horses well. It is very difficult to even get your foot in the stirrups not to mention getting them out in a hurry.
I know that you have all of your warm clothes listed and ready but do not forget to bring some warm weather clothes as well. It can get very hot in September. Also remember to leave one set of clothes out and a jacket for you to wear while riding in and don't count that as packed weight.