We are also going on the Alaska cruise tour, Aug. 16th. I have a couple of thoughts for you regarding the packing of clothing.
I have just re-read the Princess luggage instructions for the 'land tour' and it permits the small carry on that your descibes AND "you are allowed one piece of "Travel with Me" baggage PER person. This bag is picked up from your room and sent directly to the next overnight location. This information in in the Alaska Cruise Tour Answer Book, pages 22-23.
So, you will have the small bag with you on the train or bus and the larger suitcase will be sent to the next hotel location, but not available while on the train or bus.
You can pack a little more. One suitcase PER person.
As far at the jeans on non formal nights I am sure you will get other answers from other crusiers. We have made a choice this cuise not to take any formal wear. On past cruises we have had talemates that would arrive in a jacket and open collar shirt for formal night and the lady in a blouse and pants and never a negative word was said.
I believe it is how comfortable YOU feel in the enviornment of the formal evening. We are going smart causal this cruise.
I don't think Princess staff will stop you from going into the restaurants in jeans. The fashion in the general public now is a blazer with jeans. Again, I think it is how comfortable you feel in the jeans. If you are worried that everyone is saying, "oh look at the lady in jeans" and it bothers you wear slacks. If not, be comfortable and wear jeans.
No holes or worn out knees please. Ha!