Before you do anything, start with a backup. Use your iPad for backing up your stuff, if you cannot use your Mac. You talked about Dropbox. If you have a Dropbox account, duplicate your important data from iCloud. Do not place your eggs in one basket. No cloud is safe on its own. You need a backup of your data.
Yes, you can. You should try it on your Mac. If it really does not work:
Follow the instructions starting at “Remove an iOS device, Apple Watch, or Mac you no longer have”. Do this only AFTER you have created a backup of your data.
If you have removed your Mac from iCloud (AFTER you have found a solution to back your stuff up), you can do the restore setting up your Mac as a new device. Then you can sign in into iCloud and everything should repopulate from iCloud. If not, you have your backup.
After you have succeded to restore your Mac, PLEASE PLEASE PLEASE start with doing regular backups.
Get yourself an external drive and at least turn on Time Machine (and disable optimize Mac storage) so that you have a real backup. If you do not want to use Time Machine and if you want to have a bootable backup, consider using tools like Carbon Copy Cloner or SuperDuper. Given the fact that you only do have one Mac, an external SSD with a bootable clone you have created with Carbon Copy Cloner is a security net really nice to have. Just saying…
And wait for the Fall… The heat will be gone then and Catalina will just work without breaking your Mac.
And if you still want to jump on a beta again (I would not recommend that given the fact that your Mac is your only Mac), definitely create a backup beforehand to be safe. Because you have experienced it yourself: with betas you need backups to be safe.