I received a Samsung G2 2.5" 320GB that the customer had dropped whilst switched off. The drive is a HM321HX with USB interface. Drive is clicking.

Opened the drive and everything looks good inside. 1 platter 2 heads. Removed HSA and inspected under microscope, sliders look fine. No visible damage that I can see, filter is clean, top side of platter is fine, motor spins up normally. I have 2 donors, both HM321HX drives manufactured very close to the patient date. After 2 head swaps with both of the donors the drive is still clicking. I put the donor heads back into the donor drives and the donors come ready, so the donor heads are still OK. Putting the patient heads into a donor gives GrayErr's so the patient heads are faulty.

The screw holding the platter to the motor is so extremely tight that I can't get the platter off to give a good inspection of the underside of it, and try it in a different HDA.

All that I can imagine is that the underside of the platter has some damage that I can't see.

Is there something I'm missing here that I could give a shot?

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Well, even if your drive has some platter issues, they are not to deep, or else it would damage the donor heads.
But you also just said you return donor heads into donor drive and it reach ready. But have you made any reading and writting with them to check if they are still reading and writting just fine?

The only thing I can think off is that head are not compatible. Can you check both patient and donor under microscope to be sure.
Or else it might be that SA area of platter is damaged.
Have you tried hotswap?