I have tried all the usual security measures: Switch off firewalls, reboots, cache clearing, allowing installation from Developers and Apple in Prefs>Security, etc. Is there anything else that I may be overlooking?
Two things you did not specifically mention:
1./ Download the version you are trying to install direct from eMC
2./ Run the upgrade with eMC closed (exit eMC before running upgrade)
This often helps with Windows, I assume the same will hold good with MAC
The installer contains a log (menu Window/Installer Log). In the log window, you can change the detail level by a drop-down. You can explore it or paste it here (choose Show All Logs level so that we can see the context).
If that still doesn’t work, drag your current eM Client app from the Applications folder in the Finder to the bin and then try install again. It won’t delete your existing user data doing that.
If that still fails to install on your Mac, you might then have a possible eM Client database problem in which case if you have an eg: IMAP, Exchange, Office 365 or iCloud (non POP) account and nothing in eM Client Local Folders, I would suggest next to go to the following eM Client hidden database folder”
“Users\yourusername\Library\Application Support\eM Client" , and rename it to eg: eM Client-old and then try installing eM Client as new setup either via the automatic account wizard or manual setup depending on what your mailbox provider supports. All your messages and message folders etc will update from the server once eM Client connects.