I imagine that recruiting back-seaters, for what would be a very limited front-line FJ service would be problematic. You could use chopped pilots with the promise of another go at the front end when they have a little more time. Been done before
Added to that, you need to train the new Navs, using current Navs, which would just make the current problem worse.
O-P, your second point is true, at ANS in the 80s we had about 95 instructors and 140 students (at a guess). Over a 3 year tour one nav put out 4 navs.
On your first point, chopped pilots were a poor training risk for nav. My batch had an 80% failure and the one who passed was streamed FJ but unsuitable for Tornado.