I managed installing two separate instances of CLT. For some reason, if I do not (1) use the system '/bin/cp' and (2) copy the parent directory '/Library/Developer', compilations will fail. Now submit /bin/cp -R /Library/Developer ~/Library/;sudo xcode-select -s ~/Library/Developer/CommandLineTools and the new instance of CLT should work. At which point I can update the old CLT to version 14.