The only reason F Sqn exists is because of the fact that reserve training peaks big time in the summer, and falls away to almost nothing during the rest of the year. The huge number of students in-house at peak causes a huge burden of administrative and logistical work that can't be managed by a single Sqn. If it weren't for that big peak however then we wouldn't need F Sqn at all- B Sqn could just do all the trg by itself, as used to be the case before 2000. In other words, we have a Reserve Trg Squadron in order to provide a surge capacity when required: F Sqn goes to minimum manning in the winter precisely because there is no further need of it until the next surge comes along.
There is nowhere near enough demand for reserve courses in the winter to justify the money and manpower it would cost to keep F Sqn manned all year round, and the two spots per B Sqn course you talk about can and do get filled- usually when they go vacant it's because of a lack of applicants. Even if we could find the staff to run the Sqn through the winter it wouldn't make sense to do so- B Sqn is already there doing this job, and running F Sqn "just because" would be redundant. Besides, there are other positions open for those who want jobs, and those positions usually have a higher manning priority anyway i.e. tours, callouts with operational units, etc. As for ramping up B Sqn to run more courses- that's an entirely different matter, and one on which I'm not qualified to speak, except to say that some reservists are already on contract there as instructors. If they wanted more, they might be able to find them, but augmenting B Sqn with reservists would still be far less manpower intensive than keeping F Sqn through the winter.