Hello,
the uninstallation problem is due to bug in Monterey, we have reported and further escalated the issue with Apple. Currently the only way how to uninstall it is to start Terminal app and in it run
sudo /Applications/Avast.app/Contents/Backend/hub/uninstall.sh
This will ask for your password.
Regarding the shields, we officially do not support beta versions of MacOS, but the fix should get out in the next release of Avast.
Kind regards,
Ondrej Kolacek