After recovering the Windows MBR I either used a Super Grub stick, or just installed another Linux onto the partition where MX17 had been installed, to get back into my other OS's:
- When I booted using the Super Grub stick there was indeed an entry for MX17, but it would not boot. I could boot into the other Linux desktops using the Super Grub stick however. Running update-grub from another booted linux did not help me boot into MX17 afterwards, although I don't remember if update-grub generated an MX17 entry or not. It just didn't help me get into MX.
- Instead of using Super Grub, which didn't work, I just installed OpenSUSE Leap, for example, onto the previous MX17 installation. Thereafter every OS could boot, using BIOS mode. I checked each booted OS separately to see that they all were booted in BIOS mode.