Well said Tourist. The RAF gave up Nav/WSO training because there was no justification for keeping it going given the OSDs of Nav/WSO capable aircraft. There is no way (in my opinion) that a restart of the RAF Nav cadre could be justified if (and only if) the MPA role is reinstated. As an ex "Manner" such a small cadre would not be sustainable in career terms. So, if officer back seaters are required they will have to be supplied by the RN FAA, who can offer a full career structure to the Observer cadre.
For CM re compensating reductions (or not). This is the role of SDSR. If MoD want to reintroduce the capability it will either be at the expense of something else or they will have to convince the Treasury to agree to an uplift in manpower. I am sure a similar arguement has/is been going on re manning the 2nd QE class after the PM announced, and continues to announce, we would run both. I suspect an RN uplift in manpower is in the offing in SDSR, possibly at the expense of the other 2 Services.