Upload & Sell: On
Ok so I went back and re-read your OP and put some thought into your kit.
I cannot speak to Bhutan having never been there but I can speak to pretty much all of Nepal having been to Kathmandu, Lukla, Jiri to Everest Base camp and everywhere in between, Pokhara, and Chitwan National Park.
1st and foremost the electrical situation is dodgy at best. There are rolling outages in Kathmandu and I would plan on being without power for at least 12 hours a day. If you are staying in Thamel the tourist ghetto they will have power during "prime time" aka the afternoon and evening. The power supplies are decent in the nicer hotels but I would beware of bare wires, shorts, and just about every electrical nightmare you can imagine.
I am not sure if you are trekking but power along the way is decent (by decent I mean you can find someone with it). The only benefit of having the larger DSLRs is that the batteries last WAY longer than the smaller cameras we took.
Trekking: Hopefully you have a porter(s) to help carry your stuff and a good guide if you are going to trek. Carry as little as possible. I am not sure of your travelling or trekking experience but it is hard. You will have so many other things to worry about, that the camera situation (and weight) will be way down on the list. You will no doubt see thousands of people with huge cameras and if you are like us and take the small ones with go "damn that is a lot of extra weight to carry around!)... if you here a Sherpa call you a "bonduki" you know you have to much camera stuff!
In the case of trekking I would ONLY take the 5d3 and 24-105. For 99% of the trip that is all you will need unless you are a bokeh feign and need a faster lens. I might also take a small fast prime because you will encounter many situations inside where you will want to take shots and f4 is not going to cut it.
Not sure who you are flying with but if you have a knowledgable guide (you said you booked through a service) they will help you get anything anywhere no matter how much it weighs.
Remember this as well. If you are staying high in the Khumbu or anywhere above 14,000ft it is going to be hella cold no matter when you are there. We almost always had to sleep with all of our electronic stuff in our sleeping bags because we would have ruined the LCD screens and batteries leaving them out in the sub zero temps. My wife literally hugged her Mac Book Air at night so it did not freeze and I ruined a few batteries by leaving them out.
You will most likely always have a small backpack with you at all times with your day gear in it. Jacket, pants, extra layers, hat, gloves, something to cover your face, medicine, sunscreen, water, etc. This does not leave a lot of extra room for camera stuff. It was nice to be able to throw the X100 over my neck and put an extra battery in the pocket and be done with it.
Dust: It is everywhere and it will COAT your stuff. I do not mean that as a joke. You will literally get coated in dust and pollution. The pollution is not as bad as Beijing but it is serious stuff.
#1 thing I can tell you is to get something to cover your face and use it 247. Also make sure you get Tinadazole as soon as you get there. That is the medicine for giardia. Giardia is airborne there because of the dust and even a single drop of unfiltered or unbottled water and you will have serious problems. Even the most in the shower is bad. Most of us developed a technique whereas we were not in direct contact with any water mist. My wife and I were in Nepal for the better part of 3 months and we both got Giardia quite bad a number of times. The Tinadazole will help but it is good to familiarize yourself with the Wilderness Guide to Medicine handbook before you go so you can pick out symptoms of different things you may catch.
Drink drink drink... water that is. It will be dry and if you are going up in the mountains the #1 problem people have is AMS. Nothing ruins a trip like Acute Mountain Sickness.
Extra memory cards. Take them.
Do not trust any of the camera shops you see in Thamel or anywhere for that matter. Fake stuff, 2nd hand garbage, and scams. The only thing they are good for sometimes is getting a passport picture.
PM for more info.