Nice! I've considered the XMG1915-10EP as an almost perfect replacement for my aging GS1900-10HPs, except for the bootloader problem. Thanks for bringing some hope!
I assume you've already seen @andyboeh 's great work on the GS1920 series, which also uses Bootbase? If not, then look here:
I guess @plappermaul 's workaround for the slow console in comment #31 might be useful for tyou as well? At least as an idea. Different system will likely need a different , I guess.
Don't think I can answer any of your questions unfortunately. But you should be aware that the port declarations do not necessarily make sense. At least IMHO. The SoCs can do some phy management offloading and this has found it's way into the drivers in some very confusing ways. This is probably most obvious for the SFP+ ports which I assume are directly connected (i.e. not via some phy) to the SoC in this switch.
Do you know which phys are used here? If phy drivers is a problem, then it might be easier to get networking going with SFPs while you work on the phy driver.
And last link for now: @plappermaul has also a very nice collection of leaked docs:
Lacking some of the 930x details unfortunately, but combining the 931x documents with @svanheule 's register database makes it possible to connect some dots.
Did you ask Zyxel for GPL sources, BTW? They are among the best on GPL compliance and usually produce working source repos as long as the request includes a serial number. Bootbase makes it a little less useful, but still very interesting. Please make the dump available when you get one.