You have some inaccuracies in your assumptions. The removal of the gun in the Navy F-4 had no effect on the Kill/Death ratio. In fact the USAF F-4 had an internal gun... What changed the Kill/Death ratio for Navy F-4s was the creation of a Fighter Weapons School (Top Gun) in which tactics were improved upon and instilled in Naval Aviators.
Well then that is just poor tactics.
Only the F-35B is missing an internal gun. That was due to not having enough room to have one. As far as single engine goes, you mean like the AV-8B, A-7, F-8, A-4, and the list goes on.
That is already an option for the "B" model, that is designed for A/G attack or CAS missions in low-threat environments.