It is possible they're unrelated. I changed the plugs about 6 months ago, they still looked good when I pulled them the other day to swap them around. I'm sure I've got a compression tester kicking around somewhere, I'll do that when I have a minute to be sure.

Compression test was fine:
Cylinder 1: 150
Cylinder 2: 140
Cylinder 3: 138
Cylinder 4: 138

Also believe I remedied a coolant leak (thermostat housing, we'll see if it's actually stopped) and will be changing the oil later today. Cylinder 3 misfire still persists, and if it continues after the oil change, which I think it will, I'll just have to order some injectors and the pressure sensor. I figure it's gotta be one of those after process of elimination.

I'm having both the cyl 3 misfire code and the high fuel rail pressure code still. I need to do some more research, but it seems like you only need special tools if you want to re-use the injectors but replace the seals.

It seems a little debated, so I can't find a straight answer anywhere: should I replace all 4 injectors while I have the fuel rail off, or should I just replace cylinder 3 and be done with it?

OK, so update. Sorry, long post incoming.

I have not installed the new injectors, I wanted to rule some smaller things out first. Fuel commanded and measured matched, fuel pressure was good until it went into limp mode, and I also had an electrical issue, which I previously thought was unrelated, it's posted here. Basically, the interior electrics freaked out when it rained or I went through a car wash.

I cleaned the VANOS solenoid, changed spark plugs again to NGKs (I had an off-brand when I last did them), and had no real change. The car at this point was in limp mode pretty much immediately after starting, I figured the HPFP failed again, which was confirmed by a CEL P3091 and the fuel pressure never getting above around 80PSI, even before it went limp. I also had a CEL P0420 for catalytic converter/O2 sensor, but I think it was just a product of being driven in limp mode for a week or so. I replaced the upstream O2 sensor anyway as they're due at 120k and it was a relatively cheap rule-out. I pulled the valve cover and inspected the timing chain/tensioner, no real problems. The plastic was stained very darkly, so I'm pretty sure it's the original chain, but there were no pieces missing and everything seemed intact and tight, so I closed it up after snapping some photos.
I had pretty much decided to slap a new HPFP in it just to get it sold and out of my hair, but as I was cleaning up some aftermarket wiring from the previous owner, I found a loose grounding point under the driver side kick plate, and when I moved the attached module, the dash and interior lights freaked out just like it would when it rained. I tightened it up, got the new HPFP in from warranty, and now it seems to be working perfectly. I am still worried that the car will chew through this fuel pump like it has for two others already, but there are no CELs, no stuttering, no misfires, and no limp mode.

I'm trying to find a way to see if this badly grounded module is in some way related to the fuel system, so I can confirm my suspicion that this was causing the fuel pumps to fail early. I'm pretty poor with car electronics past 1999, so if anyone knows what this module does or how I can find out, it would be greatly appreciated.

Wow wouldn't be surprised if the ground was the issue all along. These cars are very picky with electrical gremlins. Fingers crossed that you fixing the ground has resolved the issue

Sad news, after 5 months of everything working as it should, I had another fuel pump fail. This one had no warning whatsoever, I was on the highway, felt the engine stutter, and it was suddenly in limp mode, fuel rail pressure reads 70-80psi straight after startup. There were no misfires and only a CEL for the cat (still). I checked the previously loose grounding point, and it's still tight, so I again have no earthly idea what issue could be claiming so many fuel pumps.

false alarm, I went to replace the pump again and found out the electrical connection just rattled loose. reinstalled and zip tied the connector and all is good. Phew.

I have exact the same problem on a 2014 JCW with N18 Engine. I already changed the fuel pump. Was your engine the N14 or N18? Because the fuel pump from N18 normaly have no problems. From N14 its more often the problem.