See:
http://archicadwiki.com/OsxMountainLion
and see if the section "Installer is damaged and can't be opened" helps - temporarily setting the System Prefs > Security gatekeeper to allow "anywhere" while you run the installer. (Be sure to switch your security back later.)
Once it does run, you might get the Java warning in the same article.
At this point, since you've installed another copy, you'll probably have to just drag the AC 12 folders (both) into the trash and reinstall one more time... Even though the various bits and pieces in system preferences/etc will not be erased doing that ... they should all get overwritten when you reinstall.
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB