That is basically the same as the proposed skipping of B in navigation. It does however require the ability to put "make document look like X" (where X is the old state) as a single undo stack entry. If your app design allows that, great, squashing all of B into a single entry would work fine.
Such an implementation would be: if you go back and try to make a change, it moves everything from the redo stack to the undo stack (to return the the "present"), adds one entry in that represents atomically changing the document to match the old state you were looking at, and then finally adds the change you attempted to make as a new entry in the undo stack.
Hmm, I wasn’t thinking of implementing it as an undo stack at all. Anyway, the main difference I’m thinking of is UI and mental model we present, which I think could be a lot simpler here.
Such an implementation would be: if you go back and try to make a change, it moves everything from the redo stack to the undo stack (to return the the "present"), adds one entry in that represents atomically changing the document to match the old state you were looking at, and then finally adds the change you attempted to make as a new entry in the undo stack.