How can I safely pause/stop Win10 chkdsk & is HDD on its last legs or can it be saved?

So a little background to my current predicament. Over the weekend (Sunday, I think), I logged onto my computer and tried to run some web games, as I do every day. I started to see some major performance issues with the PC practically completely locking up, and then I noticed that this wasn't limited strictly to when I played the games (which were all embedded in browsers and not particularly taxing). Checking the task manager showed that I was at 100% disk usage. I wasn't particularly aware of it, but I had the Win10 Anniversary update downloaded and pending on my PC. I opted to install the pending updates hoping that clearing out the downloaded update files would help bring things back to normal. I waited a good 90-100 minutes or so for everything to install, but the problem only got worse after that. By Tuesday, I couldn't even get Firefox open without it completely tanking performance. I tried my best to get into safe mode and uninstall Firefox with a fresh reinstall. I was at least able to get that running again, but occasional 100% disk usage issues persisted, namely any time the HDD was being accessed for more than very simple reads/writes (big downloads, opening up programs with large user profiles, etc.). Checking the performance tab of the task manager during these times showed almost no read/write activity. Any activity that showed up was 0-100 kb/s of read/write at best. Nothing that would tank performance like that.

Flash forward to last night and I see that Windows had a notification to reboot my PC to fix detected disk errors. Seeing as it had gotten to the point where Windows was asking me to do it without me even telling it to check, I opted to just let it happen while I slept last night. I rebooted through the notification and let it run chkdsk. It was working pretty slowly. It quickly jumped to about 4% and then didn't move anymore. I went to bed to let it run. I'm under the assumption this would be a chkdsk /f, though this isn't a function I've ever had any experience with.

So here's where I am right now. When I woke up and came down about 8.5 hours later, it was sitting at 28%, and I knew I was in trouble. Checking on it about half an hour later, it had moved to 29%. About a half hour after that, it jumped to 39%. I decided I'd be okay to just let it run, no matter how long it took, even if it was working very slowly. Over time it moved to 40%, then 41%, then 42%. I went upstairs for about 15 minutes at one point in the early afternoon and when I came back it had completely reset, now sitting at 5%. Within a half hour or so, it did move to 6%, but it was stuck there for a few hours (3+?), with no progress even though the processing icon on the screen was still spinning. In the hour or so since I've gone upstairs to do some research on an old Vista machine (kill me), it's jumped up to 27%.

So I need to know a couple of things:

• Are reboots prior to reaching 100% normal during chkdsk or chkdsk /f?
• Should I let this continue to run while it seems to be stressing the drive? There are no sounds being made by the hardware (except the fans) most of the time, but the processing light on the front of the tower remains constant.
• Is there a way to safely pause or stop the process so I can at least get back into Windows?

Even if I could get the PC reset, soft or hard, I assume Windows has it scheduled to continue to attempt runs until it no longer recognizes a problem. I think it's supposed to offer an option to press a key to skip the scheduled chkdsk, but unfortunately, my USB keyboard does not function during boot. Looking at the back of the tower and going by the mounted orientation of the motherboard, there are two USB slots at the top. Some Googling shows that these might be USB 2.0 slots, compared to the four further down that might be USB 3.0? Apparently in situations like this the BIOS might allow only the 2.0 peripherals to run during boot, but not the 3.0? I'm not sure where the documentation for my motherboard is, sadly, so I can't confirm which USBs they are. There's also a PS/2 slot next to those top USB slots, but I'll need to take the keyboard from this machine to use it. I can do that if it comes down to it.

Is it safe to plug/unplug keyboards to/from these slots while in chkdsk?

Lastly, what options do I have for the HDD itself? Should I try to roll back the anniversary update with a system restore? Should I plug it into this PC as a secondary drive and grab my most important files from it before buying a new one for the other machine if it's a goner? I've never tried that before, so does it even work that way or will it try to reformat or some other nasty thing, especially with a different OS installed on each drive? And if I did get a new drive, what about my OS? IIRC, Windows 10 installations are tied to the motherboard, so could I get an installation disc/USB and a new HDD and boot that way? Or is something else required to even get the PC to recognize that media for OS installation?

I'm open to any and all advice here. I don't want to cause any permanent damage to the PC, but it's clearly not in a good place right now and I'm worried about whether or not these disk speed issues are causing permanent damage, if there wasn't already permanent damage. Thanks in advance for any help that can be offered and I'll do my best to keep up with the thread every hour or two, answering questions if asked.

(EDIT: This post is huge. Sorry for all the reading.)