OK, guys, PROBLEM SOLVED!!!
What I had to do is flash the 7.3 usb image via USB, fearing I would lose multiboot capabilities. Luckily I did not.
What seems to happen is this:
It looks like flashing the usb image and deleting the current partition table the latest OpenATV 6.4 creates if flashed via USB, fixes the multiboot with non OE-A images.
The good thing is that flashing an OE-A image after that in any slot, will retain the table, and compatibility. The only issue is that you cannot boot Mode12 from within OpenATV due to the differences already explained, but in all images the image list is now visible, so no problem for me.
Now, even SDG8 (Zeus) is working properly.
So, long story short, the distribution has nothing to do with the incompatibility, but OpenATV's workarounds do!
Nice things to know, I learned a lot today!
Thank you to everyone for your patience and suggestions!
There is no reason why any of the OE-A images will not handle bootmode 12 when OpenPli STARTUPS are in use - the code to handle the multiboot change and different STARTUPS has been there for the last 3 months - if it doesn‘t work in OpenATV its a bug in their implementation
Edited by twol, 22 June 2020 - 15:21.