Some suggest using the EFI boot menu, and not Grub. Is that a good option to avoid problems with Bitlocker? I wonder if this is the option some calls systemd-boot. Is this an option during OpenSUSE installation procedure?
If you have full access to Windows administration, you should be able to see exact Bitlocker configuration, in GUI or using manage-bde command, there is no need to guess.
You will need to have your recovery key before you and enter on startup to boot Windows, having it in AD does not help.
Anyway - when using TPM I would say that choosing OS via your BIOS boot menu is the safest method. Using standard chainloading from grub will prevent automatic unlock. Leap does not integrate systemd-boot yet.
Whether you use shared ESP or dedicate ESP to each operating system is irrelevant. Just be careful to not format shared ESP during installation. As one of your links recommends, make sure to have bootable Windows for recovery.
P.S. it sounds like it is your corporate system. Are you even allowed to do what you intend to do? Besides, on my previous work system I could not boot from USB (I am still curious how it was achieved).
What do you mean “included”? Binary should be already available, but it is not integrated in installer/YaST/kernel updates etc. Only recently basic support in update-bootloader was added in Tumbleweed, which at least makes it possible to manually configure systemd-boot as default.
I am pretty sure nothing will change for Leap 15 and nobody knows what happens next.
I am not sure I understand it. You need to install Linux bootloader on ESP anyway, be it grub or systemd-boot.