I read that as “any mixture that is mirrored by default must remain mirrored, but any fixture that is not mirrored by default may be set to mirrored; hence, enabling mirroring is allowed”.
Also, adjusting light types isn’t permitted either, nor is changing materials in light slots for headlight, taillight, and indicator fixtures if I’m not mistaken.
As for engine paint, I’m assuming we can choose any design we like, but all of the actual paint slots must be set to default.
And with a total ban on fixture layering in place, I would interpret that as either “all fixtures must use their default layering settings” or “all fixtures must have their layer set to 1/4”.
Finally, even though it hasn’t been explicitly stated, all advanced trim settings must remain unchanged (and justifiably so - these options were never implemented in Kee), but I expect window transparency to have to be set to 0.