We did our first Alaska cruise the week of Aug. 30th on Celebrity Infinity and it was wonderful. I think since you lost that amount of weight, you deserve to treat yourself to a few new dresses, what an accomplishment!! There were 2 formal nights and yes, you would be fine in strapless and pack a shawl in case you get chilly, you would need a jacket if you want to go out on deck, but for inside the ship, that will do nicely. There are also 2 semi-formal nights where you'd need a nice dress for or a pants outfit. For the remaining 3 casual nights you can wear the linen dress you mentioned, that would be fine and also pants if you want.
We were very luck with a week of just about perfect "Alaska" weather, only 1 afternoon of rain and that was in Juneau and after
we did the whale watching excursion with Capt. Larry, which I would also recommend, it was very nice to get up close with the whales, and yes we did see plenty.
You may want to read through the review of our cruise on the homepage here of Cruisemates. It was a wonderful cruise, here is the link to the review, be sure to look at the pics at the bottom, hope it will help you get ready for your Alaska cruise:
If you have any specific questions, feel free to e-mail me at firstname.lastname@example.org