From what I have seen,all Princess cruises go to Glacier Bay. The other lines, Including HAL may or may not. I read that Princess has top priority for getting in.
If you end up in Seward, definitely take the train to Anchorage. It was our 2nd favorite excursion since there was so much beautiful scenery, and we saw more wildlife up close than anywhere else on the entire cruise. Since Princess is going to Whittier next year rather than Seward, I'm not sure of how much of the trip would be eliminated. Also, be sure to plan a helicopter landing. That is the best, most awsome escursion and is not to be missed even if you have to economize elsewhere. I would book a balcony too. There is beautiful scenery to be seen practically the whoe way up.
We went in mid May and it was cold, but some friends went the same time the year before and had 98 degrees in Ketchikan. So what everyone is telling you is correct. You can't predict weather for Alaska.