IMHO I don't think non-1901 prices will rise sufficiently to match 1901 prices in the next 3 years but yes 1901 is fixed. But the 5 year term is a legally binding committment and was a punt as they were (mainly) signed when we were in L1.
Fixed for 5 years ie another 3 to go. I dread to think what the increase will be in 3 years time though. I bought it when we were in L1, pre-Barber and it was in my opinion under valued.
I don't understand why you paid 5k up front. The upfront bit is the 1901 membership fee which would be 3k at most for your 3 tickets. Were you asked to pay more?
It varies. Top dollar are the "Platinum plus" seats at £125 a month plus VAT, Platinum is £99 per month plus VAT. Gold are less. Payment is for all 12 months of the year.