Sorry for the late reply. I was off ST for a few days.
I have not gone down Glandon, only down Croix de Fer. it should be fine. I did not find the valley to the base of Telegraph that busy in the 3 times I have done it. There is an autoroute in parallel going to Italy so that takes care of all the major trafic leaving the Route Nationale pretty safe for riding. Its easy to get to the base of Telegraphe.
Just a word of caution....depending on the time of year the town of Valloire after Telegraphe and before Galibier could be totally dead, as in nothing open and everyone on siesta. Depending on when you start your ride, you can hit Valloire in that dead zone and there is NOTHING to the top and over Galibier to the Lauteret pass. So I begged a lady to open up her Pizza place make me a pizza...i ate two slices and had her wrap two more in tin foil that I put in my jersey pockets and a massive bottle of coke and used that as my fuel to the top of Galibier. Its late in your day of riding and if it is hot and windy, or worse yet, cold, windy and pounding rain, you have a Ventoux like climb, 4+ hours into your day with no Oxygen once you get over 6000 ft up to 8800 ft.
Also one more thing.....it should be "easy" to ride down from Galibier to Bourg D'Oisans but if you start your day's riding late in the morning and end late in the day, you are 95% likely to have a massive headwind blowing up the valley pushing you back if it is hot day....hot air heats up from Grenoble and blows westerly all the way up to Galibier in a wind tunnel thermal effect.
I am actually thinking of going myself in June, July or Aug. I just don't know when as I have a tech startup (
http://www.bluwave-ai.com) and there is a lot going on with investor fund raising and customer delivery stuff over this summer