Thanks for checking out my report.
I experienced temperatures that were generally in the high twenties with maybe a couple of days into the low thirties. The good news in regard to the temperature is that there are plenty of great spots to swim most days. I used a three season bag along with a three season tent, it probably got down to single figures overnight when I was up high.
Funny you should mention stealth camping as that's what I was going to do, unfortunately my mate doesn't like carrying much gear so that meant that I was stuck at the refuges. There are generally heaps of places you could camp discreetly between the refuges, some of the spots are stunning. The two things to be aware of are that the wardens that run the refuges work some kind of a concession deal with the parks, ie they collect the camping fees in return for being able to sell supplies to the trekkers. Therefore you would need to be sensible about leaving a refuge in the late afternoon if the next one was a six hour walk away as they may be awake to your cunning plan. Having said that I always found the mood of the wardens brightened up considerably when I bought something in their small shops as they weren't really interested in my camping fees, I don't think that they are making much from independent campers. The second issue is one you have everywhere, be discrete and leave no trace. To sum it up I would do it mostly stealth camping with maybe the odd night at the refuge's.
I'm sorry I can't recall exactly how much the meals cost, it wasn't totally ridiculous, and you generally got two or three courses. Which brings us to...
Supplies. As I mentioned most of the refuges have a small shop selling the basics, you know, wine, beer, cheeses, bread and sausage. Quite a few also stock some dry foods or tinned food (not as bad as it sounds as you can normally deposit the empties in a bin at the next refuge). The only town with any kind of a decent shop is Vizzavona, roughly halfway through the walk. I didn't notice any supplies of freeze dried or similar, but I enjoyed dining with and eating like the locals.
The walking is very exposed to the sun, best idea is suss out the forecast for the next day and plan your walk accordingly. If it gets to hot in the middle of the day plan on spending a couple of hours swimming. Likewise if the weather takes a turn for the worse consider a low variant. The GR20 has a bit of scrambling but it normally has some kind of protection on the dodgier sections, I didn't find it to bad. As far as comparing it to Nepal its a different kind of experience, the culture is completely different and even the topography is different. I enjoyed standing on a snow covered mountain watching the sun set into the Mediterranean, not to mention the beaches after the trek. Not that there is anything wrong with Nepal, that would be awesome too (first world problems, hey!).
I used the Ciccerone Guide the most but also had the Trailblazer Guide, either one would be adequate. There is a good forum all about the GR20 that I found very helpful http://corsica.forhikers.com/forum
the site was handy when I was trying to work out conditions on the ground over there before leaving home.
I hope this answers a few of your questions.