I agree with you; sounds like you've uncovered the root cause of the addin problem. I don't have a good suggestion for the 2019 installer problem, but here are some bits of information that might help.
The files that you need for running macros in FeatureCAM are stored on this folder:
C:\Program Files\Common Files\Autodesk Shared\FeatureCAM
You could make a backup copy of that folder while you have a working 2018 and stash it somewhere until you get 2019 installed correctly. Then at least you wouldn't have to uninstall/reinstall 2018 every time - you could just copy the folder back. If you try the 2019 install again, you could check whether that folder even exists after the failure, and if so, what files are present compared to the backup.
I googled for the 1063 error ("windows installer 1063 error") and got nothing for 1063, but a whole bunch of info for 1603, so presumably that was just a typo. The 1603 looks like a very general catch-all error from the windows installer framework, and it can have many causes. Given the symptoms you're seeing, maybe it has something to do with permissions on that Common Files folder? But in that case I don't understand why 2019 has problems when 2018 doesn't.
Here are a couple of the links that list a variety of causes (there are plenty of others):
https://www.msigeek.com/715/how-to-troubleshoot-the-error-1603-fatal-error-during-installation
https://support.microsoft.com/en-us/help/834484/you-receive-an-error-1603-a-fatal-error-occurred-dur...
Maybe something in there might sound familiar to you?
@abhishek.juvekar, have you heard of anybody else getting this error during install?