I’d be nervous with 1 hr but as long as you are going to the cruise embarkation city at least 1 day if not 2-3 days early it probably would be ok to have only the 1 hr connection time.
If you have points you can use you may want to do that. Otherwise look at the prices online and compare and see what makes sense to you. I would definitely add Viking Air Plus so you can pick your flights - it is now $150 pp additional. Prices right now for this fall (I know you said 2025) are around $1000 RT for economy and $2500 for Premium Economy so it looks like a good deal in premium economy.
It differs depending on airline but I think it is worth it if you can get the 2 seat side. To me that is the big advantage as you don't have a stranger sitting next to you. A lot of posters have said that they won't pay more than $50 per hr more than economy. This would be an additional $850 on a 17 hr flight.
I've considered this before. I do not know the answer but assume that they will not come after you for the remaining $150 but I assume you paid by credit card for the $50 in which case I guess they could charge the card but doubt they would do that. I guess you could ask them?