I would leave them alone to finish their work. Evidently they didn't read the various sources :) But whatever you did worked, because they are there!

Assuming they are trying to make more bedding, I'm thinking about shoving some more shavings in through the hole. They can always throw them back out if I'm wrong.

I went ahead and shoved in more shavings, maybe raising the level by a couple of inches. Still nonstop pecking. So I added another couple of inches. Pecking stopped!

Flicker then sat in the hole looking out for much of the day. Now about ten days later she may be sitting on eggs. Fingers crossed.

So the advice to fill the nest box fully up (with sawdust), as compared to the advice to only put in 4 inches (of wood shavings) seemed to work better in my situation.