Installer hangs on "waiting for other installations to finish" despite no other installations in progress
When an installation happens a daemon installd runs. Killing a leftover installd should allow another installation to happen (and will kill any new ones that are waiting for the defunct one to finish). So,
sudo killall -1 installd
Nice aspect of this is you won't need to reboot.
sudo rm /private/var/db/mds/system/mds.install.lock- reboot or
sudo killall -1 installd
Chances are that a previous installation hung and the lock was never cleaned up.
I had the same problem for no apparent reason and found a couple of solutions suggesting deleting some files - but nothing helped (at least was not enough). Eventually force-quitting the installer with activity monitor and then restarting my Mac did it and after the restart the installation completed fine.