We know that the PCB1 circuit supplies a good voltage with PCB2, D10, D11 disconnected. You have checked/replaced the suspect parts on PCB2 and you have reconnected D10 and D11 and the PCB1 voltage is low.

I think the next thing is to again remove D10 and D11 and check the voltage on C9.
If C9 voltage is then greater than 16V then reconnect each of D10 and D11 and see which one drags down that voltage.

If C9 voltage is still low without D10 and D11 then I am getting desperate and I can't think of a good fault-finding check from here.
On PCB1, check C2, D1 and DZ1 ( DZ1 will probably be too high a voltage for your battery test but make sure it looks like a normal diode).
If those are OK, replace U1 on PCB1 and keep your fingers crossed!

Today I removed D10 and D11 and then checked voltage across C9, it was 5.8v. In case it might mean anything, I measured across the mounting pads where D10 and D11 had been, one was zero and the other 0.02vdc.

On PCB1 I had previously tested D1 good and DZ1 good as a diode. C2 doesn't exist. I will replace PCB1 U1 plus DZ1 as long as I'm at it. Thanks so much for sticking with this, I know it's a lot of trouble.

 

U1 is labeled TOP222P and appears to be obsolete status. Can you suggest a suitable replacement?

 

I don't know where you are in the World, but there is one here.

 

I don't know where you are in the World, but there is one here.

I am near Seattle and I would prefer to just get an equivalent part number to order from DigiKey if anyone has any suggestions, thanks.

 

I am near Seattle and I would prefer to just get an equivalent part number to order from DigiKey if anyone has any suggestions, thanks.

We do not know that U1 is faulty so the fault may lie elsewhere. Replacing U1 with an equivalent, unless it has exactly the same specification as the original risks adding an additional fault so I strongly recommend fitting an original part if you can get one.