Did you try using Internet Recovery Mode as I suggested previously? If there is a problem with the OS on your local drive then Internet Recovery Mode will bypass those software issues since local Recovery Mode boots from the internal drive if possible.
Instead of using the installer to unlock the macOS volume, launch Disk Utility from Recovery Mode and see if you can unlock it, then proceed with the reinstall.
I would try resetting the password again, but this time use an external keyboard (or another keyboard) just in case the internal/original keyboard has an odd intermittent issue.
Technically there are two passwords on a system with Filevault enabled. There is your account password, and there is another one for Filevault. Normally this is hidden from users, but once in while changing the user password will not change the Filevault password so the Filevault will remain associated with the older password. Try entering your older account passwords to unlock Filevault just in case this happened.
Did you save a copy of the Filevault Recovery Key when you enabled Filevault? If so, you can try using the Recovery Key to unlock the drive.
Does Disk Utility First Aid detect any issues? Since you get the prohibitory symbol when booting to the internal drive it means the OS is not correct or is corrupted. If the OS is corrupted, then you may have file system or drive issues which is also preventing you from unlocking the Filevault. If the drive is formatted with HFS+, then you could try running Disk Warrior to see if it can fix any problems, but I'm really doubtful this will work since you cannot unlock the drive.
If you do not have backups, then your only other recourse is to contact a professional data recovery service such as Drive Savers or Kroll Ontrack for a free estimate to see if they are able to access your data. Both of these vendors are recommended by Apple and other OEMs. It doesn't hurt to ask them if there is any hope of accessing your data.
If you do not need to recover any data, then boot into Internet Recovery Mode and erase the physical drive destroying the Filevault so you can install a clean copy of macOS onto the drive. Or restore from a backup.
