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?
We are going to China on a cruise in October with Viking. They use GenVisa for their Visa services. I know they are very good but they are pricey. Anyone use a visa service company that they like and can recommend?
Agree with @mwike - we are currently booked on the China Discovery 10-day cruise out of HK (Shenzhen) in Octobber. Very excited to see and experience the differences between the standard Viking and the Yi Dun! We will be in Japan the week before this exploring Japan on our own and it fit into the flights already booked to and from Osaka so we decided to cut our Japan trip short and go to China. I have been to China many years before and find it fascinating and the people are very friendly and hospital.