I also ran the shower very briefly with the cold water isolator right down to nearly closed, and ran on hot only, this was with the pump running, and it gave out hot water from the shower head, so i guess that would really confirm your diagnosis of the valve being faulty?
The pipework arrangement don't matter even though the hot comes from the cylinder they are both fed from the tank in loft so both will be same flow etc.
So that tells me you still have a air or blockage problem on the hot.
With the filters removed is the hot still slower than cold ? ie water exiting from filter holes.
Flow on cold is slighlty more, but not by much.....
I do get this squishy sound from the pump base when cold is introduced during commission but not with just the hot..
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