What's confusing me, I think is that the club scopes have parallax adjustment [on the objective lens as it happens] and I am needing to adjust it when moving from 25 to 15 to 10yds otherwise the target is really blurred, so it feels like something fixed at 100 would be terrible (although I realise this surely can't be the case). It feels like sub-50, parallax adjustment becomes massively important.
The clarity is kind of a side-effect of properly-set parallax. Basically, you're changing where the reticle is projected:
If the parallax is right, the reticle won't move relative to the target as your eye moves around; if the parallax is wrong, it will. The loss of focus is kind of just a side effect of how parallax correction works.
A lot of scopes don't have it, because it just doesn't matter: if I'm off 1moa at 400m with my deer rifle, I'm probably still well in the kill box. But it's a pretty major deal for precision shooting.
And yes, prismatic scopes have parallax. Even holographic sights and red dots have parallax, but theirs is set to infinity and doesn't really matter. However, if the maximum expected range is within the parallax range of the optic, it's effectively parallax-free: so if a scope has a fixed 50-150m parallax, and you're shooting 10-200m, then the parallax just won't be enough to notice outside of rather extreme measurement.
The reason I'm asking is, as you say, not to put a too expensive/inappropriate CF glass on rimfire - question is, other than price, how to tell?
Generally speaking rimfire scopes are small, short ranged, and are lightly built; they don't need to account for the occasionally punishing recoil of centerfire guns.
Most precision rimfire guns have centerfire scopes, though. E.g. Vortex makes some rimfire scopes in their Diamondback and Crossfire II lines, but both are 2-7x35 or something. The Diamondbacks I have on my RPR and T1x are flatly full-up Diamondback Tactical scopes, same as I might put on some gawdawful belted magnum. I (and apparently a lot of base-class NRL22 shooters) go with 'em because they're one of the relatively few FFP high-magnification scopes that leave enough in the budget to put a suitable rifle under 'em.