version-query | Zero-overhead package versioning for Python | Continuous Deployment library

I want to create an installer with Inno Setup, my first time using this tool.
What I’m trying to do is wrapping an existing installer of an existing software with a more detailed self-made installer (meaning a Setup.exe inside a Setup.exe).

What works for me after researching so far is asking the installer (Inno Setup *.exe) to run the included installer (actual software setup).
Why do I need another installer wrapped around? Because I want to give it some extra functions.

On particular thing is: I want to add a registry-key at the end of my installation, as the last step, fitting for relevant bit-system (32/64-bit). And here is where I'm asking expert-help, please. (main concern)

My problems in detail are as follows:

1. I managed to add the key using the [Registry] section of Inno Setup. However, [Registry] seems to always run before [Run] – but I need the key added after the installation (added in a regedit-path the installation itself creates), not before, so I deleted what already worked (just in the wrong order) under [Registry]. For accomplishing a reg-add after the main-install, I found the two procedures AfterInstall and CurStepChanged/ssPostInstall, and DeinitializeSetup which seems not to fit so well for my concern (but thinking AfterInstall would be what I'm looking for(?!) since nothing more is supposed to come after and I think it won't run, if the install before already failed (?!).
2. I don't know the Pascal-Syntax for adding a registry-key (string) under an existing path. I could add it under [Registry], however when it comes to the [Code]-section I feel a little lost even I did a lot of research by now about Inno Setup given functions and such.
3. Third problem is that the path in the registry differs, depending on whether it's 32- or 64-bit System. So I actually need an extra query here checking the bit-System before adding either one or another path/key (because the install of the program itself creates the path depending on the bit-version already), I found the function IsWin64 (Boolean), now trying to mix a function (bit-version-query) with a procedure (AfterInstall) sounds even for me as a beginner wrong. Plus I tried to create an if-else-query, and the compiler told me I was doing it wrong. if IsWin64 then... works, but adding an else doesn't.

So the solution in theory would roughly be something like…