is there anyway to ascertain how bad rust is, and anyway to reverse what already exists
Just done a strip down on the Bonty race and not looking good, a few holes in the seat tube 🙁
They are only little holes so i am fairly sure the frame is ok, how much metal do you really need, but if there are minor holes then other areas are probably pitted on the way as well.
wow, holes? really? if it's got to the point of rusting all the way through then i'd say you have problems...
having said that I have a knackered old Kona I use for commuting that has had 3 scary cracks in it (one of them over 1/2 way round the seat tube) for the past 2 years, sometimes it really does depend where the damage is.
your very lucky, you have the perfect reason to buy a new frame.
you have been blessed...
"how much metal do you really need"
pretty much all of the metal it came with when new.
i think that your frame is dead. My only comparison would be steel water pipes, and if you have got holes like that in a few places, the rest of the tube / pipe is likely mostly made up of rust rather than structural steel. Ideal excuse for a christmas present.