-snip- Just to be clear, there is no way to create a 0.12 era (non HD) wallet on the latest version? because then I will have to start building 0.12. Once I build 0.12 and its installed and I get the wallet, do I compile the latest version and install on top of this existing folder isn't it? so I would need to be sure to get the wallet to test properly.
I'm not aware of any deprecated config/arg to enable non-HD wallet creation to the latest version.
As for the installation, there's no specific location to install the binaries as long as you can find and launch it.
Installing over the old version should work as long as it's not running.
