Yes, 690 ohms sounds very low. It definitely sounds like there is something wrong there. However, it may be hard to diagnose, because it seems as though something has failed partway– not all the way open or shorted –either of which would make this pretty straightforward. In the absence of any better guidance, and if D12 seems to be working more-or-less correctly, I’d suggest replacing D21, as it seems like that is the likely point of failure.
If you’d prefer, you might want to consider sending it to us for analysis; we’d be willing to replace that LED for you, and figure out what the problem is otherwise.
As far as the faceplate without a hole goes, you may not need one. A common procedure when installing the rear-projection plate is to use the original opaque clock face to cover the big hole in the other side. Just use it instead of the washers on that side.
