2. Mt Rainier and Olympic are very different. Frankly, I think mountains in the Pacific Northwest are far better looking and far greener than those in Colorado. Do not miss the Olympic coast if you go. May is little too early but Rainier should not be missed. Olympic coast and Vancouver island coast have similarities but Olympic coast has more big rocks and perhaps more dramatic. Both may get rainy in May since they are in the rain shadow.
3. North Cascate National park is nice. Another nice area to visit (and should not be missed) is Mt Baker area. There is nothing like Mt Shuksan reflected in Picture Lake at sunset. However, Artist Point, where you can see both Baker and Shuksan may not be open yet in May.
4. Personally prefer to stay at B and Bs, not big chain hotels while travelling in BC. Many of them are right on water with very nice views. Not sure about air from NYC. I would think to get a good fare you have to travel outside the Memorial day window.