Perfect that you have confirmed the fuel pressure and can move on from that.
The next step should be to redo the compression test (most likely problem at this point) and then maybe actually check spark and plug at #5. You are missing something.

Stepping away for the next 10 days and will pick up with this when I return, though I've done multiple compression checks, checked the plug gap, swapped the plug, etc.

Chasing a mystery
 

.05v on the O₂ does it ever go up? it seems like oxygen sensor that’s not reading. It’s weird that only the center cylinder is misfiring. Swap your O₂ sensors or at least the connectors to determine if the sensor is reading wrong or if it's actually not getting fuel.

@OP everything you are describing sure sounds like a compression problem. Bad valve guides can sometimes not show up on a compression test.
I think number 5 makes most of the brake booster vacuum so something else to check.
There is no "normal" amount of fuel on the spark plug so thinking that the cylinder is getting too much fuel based on that is the kind of thinking that will lead you down the garden path.

This could honestly just be a faulty DME. The MS45 is notoriously problematic. The earlier MS43 was pretty much bulletproof but MS45 has all kinds of issues and can fail in a way that just continues to throw persistent, odd codes. I've had to replace more than one over the years

Ding Ding Ding Ding!

My MS43 w/EWS delete showed up while I was out of town. I plugged it in and now the X5 purrs like a kitten.

Thank you everyone for your thoughts and suggestions!
Back to lurking while I keep fixing up this beast.

I'm glad to see the follow up!


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Interesting! EWS is so easy to do. I spent $100 on a DME/EWS delete while chasing a parasitic drain. It was easy to back once the problem was fixed. It ended up being the GMIII module causing a drain.

When you scan it, does it show a different VIN?

Nope, VIN shows up correctly.

I might be interested in the injectors if they don't have a lot of miles and you want to let them go cheap ;)