Software Deployment

Hey everyone. Hope everyone's doing great.

I was wondering if, given a computer, if I could easily find out the list of software installed and who it's installed for programmatically. Is all the information available in the registry? So far, I have found out that the following key has the full list of installed application:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall
The main product and corresponding patches are best organized in the registry key below but it's missing many entries of the main product:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products
Above key also seem to contain "RegOwner" value which appears to indicate who the product was installed for, but it's not reliable. The data is not there most of the times and the value itself is missing sometimes.

1. Is there a registry key, property or any other method of determining who the application was installed for?
2. If 3 users installed the same application using "Just Me" option, is it possible to identify the 3 separate "installation" for these three users? Or, is the last successful install the one that I would be able to identify?(That may be sufficient.)
3. If some users installed it for themselves but another user suddenly installed it for Everyone what would happen? Would the installation be in semi corrupt(unclean) state? (Not functionally corrupt.)
4. What about the opposite case where it was installed for everyone but a user installed it for just him/herself?

Thanks for your help. If anyone has any info/idea to share, it would be really appreciated.
Thanks!!!!!