The protection circuit senses DC and trips the relay above a certain threshold (IIRC it's something like 200mV - 300mV). With no driver board, there's no DC at it's input, and no reason for it not to do it's job. This is no big deal but what we really don't know is how it performs with DC present in it's input. So far, based on your measurements, it should have worked - but it didn't, and it doesn't look good for the protection board.
You say it rattles and it shouldn't, so here we have at least one suspicious aspect of it. I wonder what makes it rattle and I would likely assume it's the relay's electromagnets struggling to make contact. I don't know. I never heard a protection circuit buzz before, but it sure shouldn't. We also know this board (it's been posted in this forum before) to be prone to bad/dirty relay contacts and it's generally sensitive to it's solder joints crumbling and cracking over time.
That relay is often replaced (or at least opened and cleaned) within the framework of a unit's recap together with overhauling this aging board. Did you try to tap the relay like Doug suggested? It's a good tip and if you manage to release the relay by tapping it, it would mean the protection board is just stuck on a technicality.That would be good news.
So far it just looks like your protection board is not doing it's job, or that it's doing it's job so well, that it detects high spikes of DC that we don't see on the meter. It's not impossible, but unlikely.
OL means open loop. You don't have an open loop while the transistors are in circuit. Anyway, if you had a bad transistor on the driver amp, let alone all of the above, the DBT would most likely oscillate, you'd have noticeable current draw, you could expect very high levels of DC on the gray/blue wires and there are good chances you would not be able to adjust bias. You got none of it and it appears your driver amp is good. Be patient, stay on DBT and keep it this way.