Read the Little List letter again with Ross in mind. It really feels like childish threats of a disturbed person. Ross might have flown under the radar for the reasons JeffP mentions.
But we need to stick with facts. Ross's mental conservatorship began in late 1970, not 1969. If we find out Ross was under care throughout 1969, then I would drop him.
But we also know one more fact. He wasn't listed in the Polk directory for 1969. This directory was likely compiled in late 1968, so Ross was already gone by then. So where was he? He had been in Santa Cruz for school. If he was sent to jail or care and let out in mid 1968, he might not have a reason (or means) to move on back to Santa Cruz.
I bet something happened to him by late 1968 that connects him to the Vallejo/Napa area.

