If I understand the oil supply to the top end, it is pressure fed to the left side cam bearings, but splash fed to the right side ones. If the engine was running for any time with the bike on its left side (ie tip over), that would interrupt the oil feed enough to fry the right side bearings, perhaps.
The hard black rubber in the oil strainer could be from the rubber surround (damper) of the balancer chain drive sprocket.
Excess balancer chain tension would cause the chain to dig into the rubber. The oil screen in my '08 was perfectly clean even after 28000km. The balancer chain has never been adjusted from new. (and because the spring is too long it has no tension anyway . I'll be shortening the spring a turn or two but not so much as to excessively tighten the chain)