You don't know anything about cycling Doug. The roads that are "extremely windy, hilly and lined with thick trees" are absolutely, by far the best roads to ride a bike on. This is true around the world. And yes, these roads weren't built for cycling. None are. If you think that "cyclists should take their leisurely bike to the Katy Trail as that's the reason it was built," you're crazy. The Katy trail works OK for very recreational riders, but nothing else.
As you say, this area doesn't have a street grid. Thus, the problem is that if you close even one road then cyclists will not be able to get to many others - the connections to various routes will be gone. "Someone would be insane to bike down a highway and so are these people who want to cycle on Highway D/DD/Z/N or any of those rural roads." Again, you don't know anything about cycling. That's not a put-down, it's just an apparent fact.
You seem adamant that your interest is protecting the cyclists who, according to you, could easily get killed at any time. Do you also think that anyone on a motorcycle should be required to wear a helmet by law? What about red light cameras that improve safety? These roads should be accessible to all and those creating problems, whether cyclists, or the drunks and others you mention, should have all applicable laws enforced. Some, if not most cyclists should absolutely stay off these roads and lot of cyclists wouldn't dream of riding out there due to traffic, the hills, etc. But those who want to should be able to.
Put the speed limit at 45, these roads are exponentially more dangerous as speeds go up. The vast, vast, overwhelming majority of fatal accidents on these roads involve a single or multiple vehicles and not bicycles. Cyclists are not a public health concern, even on these roads. It's ridiculous that a County rep doesn't step up and insist on addressing speeding and drunk driving, no...they decide that people on bicycles are the problem! That's just stupid.