I rang my mortgage company as in the same situation, tracker mortgage last June and two hikes already so increased by around €60 and they helped me work out that even if there is two more hikes in interest rates, I would still be not be paying as much as the one year fixed rate which would cost me €1,063, mortgage at mo is around €960
so my answers would be the following:
1. it's really up to you but I would advise ring your mortgage company and let them work out how much that would be.
2. You can change at any time with variable rate
3. Just ring mortgage company and advise over phone and they can change for you
4. Mortgage company will give you offers of 1 year, 3 or 5 or more etc.
5 & 6. What suits you and how much you can offord each month, would always start with one year fixed and at least then in a year you can see what interest rate situation is and change back to Variable.
Hope this helps!