I have had some alterations done to my house and the solar connects to the hot water tank then goes to the pool via a three way valve/thermostats. The old system included dump radiators, but do I actually need these dump rads when the heat is already diverted into the pool. (The pool is about 9m x 4m x 1.6m)

What's needed for solar thermal is a system which is fail safe in the proper meaning of the term: that is, a system which will be safe if it fails.

The question then is would that be true of the diversion to the pool - e.g., what happens if the three-way valve or thermostats become stuck in any plausible state and if the safety relies on redundancy will it be obvious if that redundancy is lost (e.g., if there are two thermostats and one sticks will there be an indication of that).

How do you transfer the heat to the pool water? If it is, as I assume from the size, a swimming pool then presumably it has chlorine or salt or something in it so I'd guess there's a heat exchanger somewhere as that's not the sort of water you want going through most solar thermal panels. Is there a pump on the pool water circuit for that? What if it fails?

Assuming an unvented cylinder, a PRV is, of course, necessary and useful but I don't think it's sufficient for a cylinder fed from a solid-fuel boiler so I don't see why it would be for solar thermal, either.

Hi Melodie, welcome. Based on my own Solar thermal experience and from the size of your pool I wouldn't think you'll have any problems discharging excess heat into it. I've no idea where you are but assuming UK, and assuming a fairly standard sized array, and not something vast, the pool should easily absorb any excess. Of course, I've no idea of your particular system layout.
You may have a combined pressure/temperature relief valve which should be attached to a tundish, with visible discharge as your safety, if your system is pressurised.