you mentioned the fact that the implementation of UEFI firmware is haphazard with every firmware manufacturer providing their own version or lack of it. You can take my Mobo (MEDION AKOYA Notebook with [reportedly] MSI Mobo) as an example for this.
If I enable CSM, any of the two Boot Managers i.e. Windows Boot Manager or BIBM would come up depending on the boot priorities setting. In the case of the HDD being first it would boot into Windows 8.1 correctly. In the case of the SSD being first in the boot sequence, BIBM would show up.
But there appears to be no need to fiddle around with BIOS settings:
I found out that with CSM Enabled and the HDD being the first in the boot sequence, the Windows Boot Manager will show up first. If I then press ESC, the BIBM Boot menu will appear. So, if I can install Win7 and Linux Mint on the SSD, everything would be alright. The boot process would be reasonably simple (text partly quoted from my thread viewtopic.php?f=2&t=1928&start=20