I noticed my D: drive was marked as dirty and some folders were coming up as corrupted, so I recovered as much data as possible, and then scheduled a chkdsk /r /f on next restart, and I've been here since Thursday (it progressed a bit throughout Wednesday):
I now need to urgently access files on my C: drive, but don't know if I should interrupt this process with a forced power off.
If you can afford the time and it appears to be progressing, let it ride.
Either it is recovering further files which you can quickly remove and save elsewhere or it is busy scuffing the disk into an unrecognizable state. If you have all you need then it doesn't matter if you let it run. I wouldn't trust it after this.
Well unfortunately if it's the bearings dying, it continuing to run will likely overheat it and you risk locking it entirely. I've had to freeze a drive before scanning it and backing up, but it's only about 50% successful.
Yeah, that would be another theoretical possibility OP will need to weigh.