Initially I thought foreign object damage, and that’s definitely a possibility, especially if those aren’t the original heads. But you’re right about not seeing damage in the head.
Overheating can cause damage like that, but I doubt it would be as localized. As stated earlier, piston slap/skirt are also possibilities. Maybe from sudden engine torque changes since it’s a manual.
https://www.ms-motorservice.com/int/...nd-causes-233/
You covered all the normal oil burning causes and I’d say the gouges are most likely the source of the oil. As to the cause of the gouges, it will probably take a tear down and inspection of the pistons to make that determination. If you do that, please post pics.
Thanks for sharing.