We would like to acknowledge our debt to everyone who has helped out, publicly and privately, with this seemingly intractable problem. In the end, we have gone with Yogi's workaround (immediately above this post) -- it fitted in best with all the other mods we had already made.

So now we can display the modify form without fear, uncertainty or doubt. It shows users which categories the link is in, but does not allow users to change those categories -- intentionally or unintentionally. The original categories are selected in the Admin validation form.

It is all we needed. Thanks very much!

Now I'm takng a break for two weeks ...