I don't recall if you posted the actual 5V supply voltage on the CPU and Driver boards. Easiest to measure across the 100uf filter cap (5V and ground are on either side of this cap).

Q1 - You are spot on - any TTL chips (which should be almost all of them on the CPU and Driver boards) you keep it on TTL. Setting it on CMOS and measuring TTL usually works fine but can give you erroneous pulsing readings.

Q2 - start at the pins on the CPU chip. You are looking for activity on the data and address pins. Lack of activity here means the CPU isn't running (could be clock, could be the CPU chip, could be program (ROMS)).
Then check that the blanking signal is going hi as it should.
Then check that the PIA input and output pins are pulsing. If the input pins are pulsing but the outputs are not, could point to a bad PIA (can also mean the program isn't running correctly).

And I second what the others have said and I suggested it in the other thread, just because the prior owner said it worked for years doesn't mean it really did. Maybe ask the person you found with a working Blackout with a system 4 board to send you closeup pictures of the CPU board. That way you can check the jumper settings. There are changes needed to the typical stock Sys 4 board to support the green flipper ROMs from a Blackout.

Here is a copy of the 6821 PIA data sheet. https://www.jameco.com/Jameco/Products/ProdDS/43596MOT.pdf

Well, that's a good sign. I don't know how to tell if the board is running correctly or the PIA is bad. If it is already socketed, not a bad idea to try replacing it.

Did you test the outputs of the other PIAs?

What about the blanking signal? You can find it on pin 37 of the 40 pin connector (and other places).

Awesome! Congrats on figuring it out!