Progress Bar Stuck on 100% on boot High Sierra

I have a Mac Mini (Late 2012).

Last month I upgraded the memory from 4GB to 16GB and installed macOS High Sierra; am currently on version 10.13.1

Yesterday (11th November 2017) I booted up my MAC at 9 am (ish) with no problems, during which time it successfully backed up twice to Time Machine at 9:15 and 9:50.

I shut down not long after this, then booted up again at about 13:10, and noticed the usual progress bar beneath the apple symbol was taking a particularly long time. I did a hard shut down and restarted, and still it took a long time, so I left it, then after about 20 (far longer than usual) minutes, it reaches 100% and stayed there: no log in screen.

I shut down and tried to reboot in Safe Mode (holding shift after the chime) but this made no difference as I never got to the log in screen.

I reset PRAM/NVRAM by rebooting and holding command+option+P+R at the chime. Nothing.

Reset SMC (unplugged, held power button, released, plugged back in) still no log in screen after 100%

I booted into recovery mode, opened Disk Utility and checked my hard disk. It returned ok with no errors found. Rebooted, still stuck at 100% with no log in screen.

This is where it starts to get scary.

I rebooted back into recovery mode, and reinstalled High Sierra (took about 3 hours), and, incredibly, it DIDN'T FIX THE PROBLEM!!(still can't believe this!!!) After installing, it rebooted, and again after half an hour the progress bar reached 100% and stayed there: no log in screen! How can actually reinstalling the entire operating system NOT FIX a start up problem? Does it not overwrite the system files it needs to reboot? I just don't understand how this is possible!?

I then went to bed and left my MAC on overnight. In the morning (about 7 hours later) still apple symbol with progress bar at 100%; no log in screen.

I booted in Verbose mode (command V) and saw lots of crashed processes "producing too many corpses"

I decided at this point to restore from a time machine back up. I didn't chose the ones from the morning before because it was after these back ups that my mac wouldn't restart, so opted for the back up the night before because I successfully restarted after this the following morning.

It took 8 and a half hours.

When it had finished, my MAC started just like it had before the problem occurred and everything was back the way it was before anything had happened.

The point is: I am now absolutely terrified to shut my mac down. I did nothing - install any new software, download anything etc - between booting up yesterday morning and yesterday afternoon which would have caused such a critical error, and I just can't believe that I couldn't boot into safe mode, and reinstalling High Sierra didn't fix it (still can't get over this), and am not going to be able to shut down until I know what caused it because I can't wait 8 and a half hours every time I need to use my MAC!!!

I read some articles about kext files, and booted in Verbose mode and saw some error messages about crashes and "too many corpses". I've also in Terminal compared the current system library with the one from the last back up that I successfully booted from and it's only found very few differences - mainly mobile assist fonts, and only 8 cash files - which is really surprising; are the files needed to start up kept somewhere else other than System/Library? I also read that other people managed to log into their macs after restoring from a back up like I have but after the next restart the problem still occured and they couldn't get back in.

Could all this have been caused because I interrupted it when it was being slow to start the first time? But why would it suddenly take so long to start when I haven't installed anything new, and there hasn't been any updates?

Would it be safe to wait for the next update and shut down then?

Is there any danger in leaving your mac mini on for a long time? (fan, heat etc)

Obviously the longer I leave it, the more data I will lose as I will have to restore from the last back up before the problem occurred which is currently the 10th of November, if it won't start again.

Anyone had the same problem and found a fix? Is it now safe to restart? I head something about Kext files in the Extensions folder and moved them but it didn't fix my problem. I can't think of anything I did between 9am and 1pm that would cause such a slow start up followed but such a fatal error; I installed High Sierra over a month ago, and installed the last update over a week ago.

What could High Sierra have done on it's own in the background between 9am and 1pm that now prevents it from starting up?

HELP!