We have just completed the GTMC and although it isn’t “mountainbiking” it is excellent! There are several stretches of very nice singletrack and the felling of isolation is incredible. We rode the route entirely self supported,carrying a tent,stove and food for a couple of days at a time
jwh-we have also done your number two suggested route from Chamonix to Samoens-yes it a fantastic ride easily manageable in a day. I can also highly recommend the Samoens area for mountainbiking,we worked there this winter and as the snow was not the best,we managed a lot of exploring,discovering some very good singletrack,considering we have lived over in Morzine for the past few years,it was a real eye opener!