catalina os update failed and now all **** has broken loose

My guess is the internal hard drive has failed. If so, then you want to stop accessing it since it will likely make the hard drive failure worse where even a professional data recovery service will be unable to recover any data. Most apps, even a data recovery app, are not able to handle the errors produced by a failing hard drive (and neither can macOS handle it even if a data recovery app can).

First, run DriveDx (free trial period) to check the health of the internal drive(s). Post the complete DriveDx text report for the drive(s) here using the "Additional Text" icon which looks like a piece of paper. Depending on the health of the hard drive and how bad the failure will determine the best course of action if you need to recover any data from it.

A lot to sort through here, but we shall start. You say the reinstall failed. What happened? Did you attempt to reinstall the same file that had been downloaded. If so I believe that the download got corrupted and cannot properly install. Did you use Wi-Fi to do the download? If so, don't. It is often too slow for these very large file. Instead either use a wired Ethernet connection or you may have to do multiple Wi-Fi downloads until you get a clean one that works.

Yes, your Mac sees the HD but since it doesn't have an operating system on it that works, it cannot be used as a startup disk. at this point I would do a new download as described above. If that still doesn't work, you may have to erase the disk, reformat it and then reinstall everything.

You seem to say that you do not have a backup in place, so that is a BIG problem if you do have to do the reformat, which will wipe out all your data. It sounds like you are now able to startup from an external disk. If so, then do so and see if you can access the Mac's HD and copy your files over to the external disk. When this all done, then get an external disk and dedicate it to backups and set up Time Machine to regularly do them.

I never recommend downgrading. It involves having to completely erase the disk and then reloading all your saved files. It's a big deal and isn't really the best solution. As I suggested above, do a clean download and reinstall. First, put the prior download file into the Trash and empty it. Give my suggestions a try and let's move forward and see how things go.

Keep in mind when downgrading, your libraries, albums, and playlists will be lost that were already upgraded, unless you kept a version of them external to the computer prior to the initial upgrade.

This is why it is essential to backup prior to any install.

It is not a question of if hard drives fail, but when.

How do I downgrade my Mac OS X operating … - Apple Community

Discusses downgrading and its pitfalls in more detail.