I understand the general rules for squads. I know what optimal is and how things should be done, but there are always exceptions to the rules. You are almost saying we must cripple ourselves for next season and take a risk on one them being fit, when they are proving the exact opposite is almost a certainty. If we are unable to get rid of either, by selling them or loaning and recouping funds that way the only other safe option is to buy another. I'm pretty certain SJR has already said as much.
It won't be an excuse come this time next year if we are in the same situation, everyone will be saying you had this information one year ago. Who is to blame? They're then going to tell people that it was because we couldn't sign a third left back in a problem position because of squad structure? No. it wouldn't be acceptable. A new LB is needed 100%, regardeless of what is to be done with the other two. It is easily the most problematic position for us right now.
So stating the Fm setup to me is not a point it's stupid. You do what you need to do based upon your needs. If that does mean selling a LB once we have acquired another so be it - but make no mistake, the priority is identifying one and buying him.