What caused this

It looks like the middle lifter never rotated at all.

Good that the filter is clean so far. Any metal sucked up will go into and through the oil pump and nick the pump gears. So I personally would not do that. Trying to flush with the pump is a bad idea IMHO; some of the tiniest particles may get through the filter and go who knows where. And solvent will just make it more prone to push stuff through.

If you are trying to avoid a full teardown..... most of the crap from the cam would drop on the crank, rods, and down into the pan. Multiple flushes from the top down through the valley, and repeated wipes of the rotating assembly would be be the best IMHO. But I doubt it will get it all, and you just hope that the rest gradually works its way down into the pan.