Also, I plan on starting the NT by May 31st in Seattle: is this too early or too late?
I once did Seattle to the NT and then headed east twice. Both times I started around May 23. Don't see how May 31st could be too late, although I don't know what criteria you might have in mind. School will likely be in session, so the crowds will liekly not have materialized. Plus, it will likely be cold and wet in the N. Cascades that early, deterring many visitors. It's only about a 4-5 day ride until you hit the mountains. Whether it's too early will likely depend on your tolerance for chilly, wet weather.
Here is some data on historic openng and closing dates of the N. Cascades Highway:http://www.wsdot.wa.gov/Traffic/Passes/NorthCascades/closurehistory.htm
As you can see, a start date of May 31st will likely not put you there before the road has fully opened. However, that does not mean you won't run into some weather. In '99 I hit rain somewhere along the climb. By the time I got to the first pass, the rain had turned to snow, although it was not sticking to the road surface. The following year there were a few snow flurries on the way up, but nothing major. Both years it poured a cold rain between Bay View and Rockport, and it was generally chilly and damp until Winthrop. If that is not for you, you might want to start later. Between Winthrop and Tonasket, it's generally warm and dry. Republic, at the foot of Sherman Pass, was on the chilly side. Snow showers crossing Sherman Pass both years. The rest of the way to Glacier N.P. was often on the chillier side with some light rain on several days. But by no means miserable.
When picking a starting date there is an additional consideration down the pipeline: The opening date of Logan Pass in Glacier. You never know what's going to be up with that, especcially if the road re-hab project is still going on. For example, this year the NPS has announced that the earliest
you will be able to ride the full length will be June 20th, even if the road has been fully plowed. The likeley reason for this is that they want uninterrupted time to perform work on the road. It's a lot easier (and cheaper) to perform road work with no traffic. If there are things like eemergency repairs needed due to avalanche damage, as was the case when I was lats there in '09, the opening could be delayed even further.
As for sleeping bags, Staehphj1 prety much covered it. Some people sleep warmer than others, and, as far as I know, there is no one standard that measures warmth. I took an REI brand 32 synthetic bag and got by with it, although I remember being cold a few night depsite having a good deal of clothes on. My current 20 degree Cat's Meow has served me well in sub-freezing temperatures.