Mickw; I think your situation might actually answer some of the other questions you posed in other threads. Why would someone buy the new 300mm II?

The image quality of the 300mm II with the 2x III might actually be usable in the size of prints you want to use.
Going with the 2x III and 300mm II over the 500mm saves you having the 7 1/2 pounds of the 500mm. In your situation only having one lens might be enough to still be workable. Still the 300mm II weighes over 5lbs.
If I were scaling back this would be my first step.

Since the 200-400mm may never materialize, it really isn't an option. It is a dream.

Next step I would have to think seriously and this would probably be my order, 300mm F4 or 400mm F4.
Then the zooms 70-200mm II with an extender, 70-300mm L or the 100-400mm L.

Good Luck and I hope you only get good news here on out from the doctors.