I would agree with bigfatipod that with the information presented, the most likely cause would be the HDD. You could have a failing HDD and the OS wouldn't always detect it. You could also have a slowing HDD and the OS would detect it. There is software available that can test every sector of the disk but for most people, the cost of the software isn't worth it if they're only every going to use it on a single HDD. So a lot of people just replace the HDD or upgrade to a SSD when it begins to show sign of aging.
High Sierra did not support fusion drives on APFS. From what I've read - when upgrading from High Sierra to Mojave (which does support fusion drives on APFS), it converts to APFS from HFS+. Did you revert back to High Sierra? - in that case, High Sierra wouldn't be able to use the APFS fusion drive.
What could also be complicating things is the Boot Camp partition, which should be on the HDD. With the conversion from HFS+ to APFS, the OS should be able to work around the Boot Camp partition, but at the very least it does add more complexity to the situation.
People have reported problems with APFS fusion drives on Mojave, I don't recall seeing this before 10.14.4, but since 10.14.4, there have been several reports of problems.
You have several options:
1) Buy a SSD to replace the fusion drive setup. Good performance, brand-name Internal 1TB drives are about $120 - there are cheaper alternatives as well.
2) Stay at High Sierra. As I mentioned, if you upgraded to Mojave, you shouldn't be able to revert back easily to High Sierra because of what it did to the fusion drive.
3) Try clean-installing Mojave. This would involve erasing the "container" and then re-installing on it. The idea here is that Mojave got confused with how the fusion drive was working in High Sierra HFS+ and doing a clean install will clear that up. Unless somebody posts with experience on how this would work, you should probably call Apple for guidance on this.
If you think there may be some other hardware issue involved or maybe the problem is Mojave, if you have an external SSD, you can try installing Mojave on there and see how it works. If you don't have a SSD to use and you don't mind spending $20, you can buy a 120GB Kingston or Crucial SSD and put it in an external enclosure to try this out.
IMO, I would not buy a new HDD to replace the old HDD in a fusion drive setup. The price difference between a new brand-name HDD and a SSD is not enough to deal with the complications that fusion drives bring.
If you can run the Terminal app and type in the following command and post the results here, that would help to understand what your current disk setup is: "diskutil list"
Regarding the WiFI, have you tried going to Recovery (Command-R when the computer starts up) and see if it works there? You can also try doing a SMC reset and see if that restores it.
https://support.apple.com/en-us/HT201295