"5)- Do we need 'bear canisters' for our food?... or is there another alternative? Can you rent them? (we want to pack as LtWt as possible)"
You have to have them. If you run into a ranger and don't have one, you will be required to turn around and walk out the way you came in....if you are lucky, you won't get a fine, also. they can be rented at the Whitney Portal Store, or at the Ranger Station.
That time of year, it is unusual for it to get below freezing much. A 15 or 20 degree bag, with knowledge of how to use them properly and extend their range, should be quite sufficient. I know experienced people who would use a 30.
Here is a link to the Upper Tyndall Creek snow survey site, that gives hourly temps. Seems like the low today is around 19oF
http://cdec.water.ca.gov/cgi-progs/queryF?UTYLooks like the snow is about 75in deep (water content about 25", and the density is about 30% right now, so about 3X the water content)