Hi Mrs K,
If your curve is progressing as it is then most surgeons would advise surgery at this point I think.
I'm very confused by your surgeon saying that your correction will decrease by 10% yearly - does he mean get better or get worse? In either case it's an odd thing to say; you can't straighten out further once you are fused, but neither should you expect to lose that much correction! A little "settling" occurs following the surgery, but once you're fused your spine should stay where it is. I would get this clarified as soon as you can.
Removal of the disc and replacing it with a spacer (to restore the height lost from the missing disc) and then fusing that level is pretty standard treatment for a badly degenerated lumbar disc, whether you have scoliosis or not. It's odd that your first consultant didn't suggest this. Your current surgeon sounds much more on-the-ball. Please don't worry about the discogram; in my experience they are uncomfortable rather than painful.
Hope this helps