Same issues on iMac 2015, with total freeze up in recovery mode - no control with keyboard or mouse so I couldn't make any choices. dsortor had the solution in my case. Hard shutdown, then hold down the option key when restarting (startup manager) I had 3 choices: MacOS Hard Drive Backup disk. I chose my hard drive and finally have my computer back! I would say the problems stem from the computer trying to restart from the install, as opposed to restarting from the hard drive. If the install is bad, then restarting is bad. If you can get to startup manager then you can choose to restart from your hard drive instead and maybe you'll get back in - good luck! I'll wait on that upgrade I think... ;)