Wow, thats a lot of questions. It doesn't sound like you've talked with a travel agent there in Canada who can help you sort this all through.
Food taste is very subjective and I've never felt that Celebrity's food was bland. One of our members here on the board (Janfred) just went on the Mercury and I think she found the ship in top shape. That being said; Princess is a good choice for Alaska also. You might want to check into a whole complete package with Princess. Meaning air/transportation to the ship/cruise. Rather than trying to piece everything together. You would definitely need to fly in the day early in order to catch the train in the morning to Whittier. You will see plenty regardless of which side of the ship you choose.
Alaska is a short cruise season. June through August are the peak months. But weather in Alaska is very unpredictable. One year I went in August and it was beautiful in the 70's with no rain. Those that had sailed just the week before had cold rainy weather. I wouldn't worry so much about the weather. Pick what works best for you and pack clothes you can layer if need be.
Most cruise lines will let you cancel shore excursions with 24hrs advance notice. I would go ahead and book those excursions that are very popular and could fill up. Some things you can do on your own. In Ketchikan there's a tour office right on the pier.