So long as the neurology produces the correct behaviors, eating the right food, running from threat, finding water, what the subject believes is of no concesquence as far as evolution is concerned.

This is true, of course.

If it is behavior, rather than beliefs that are selected for, then there is nothing to make our beliefs reliable.

Beliefs determine behavior. So our beliefs must be of a type that makes us likely to survive and reproduce. If we have reasonably accurate beliefs, we are much likelier to engage in behaviors that encourage our survival.

If naturalism and evolution are true, and R is not an adaptive state for an organism to be in, then for any one of our beliefs, the probability it is right is roughly .5

Pedantry time: choosing a probability of 0.5 is wrong. If there are a total of a hundred mutually exclusive hypotheses, and our belief that isn't based on reality is one of them, our belief should have the same probability as each of the other hypotheses. That means every one of these hypotheses should have a 0.01 probability. And if there are a billion hypotheses, each one gets a probability 1 / 1,000,000,000. This is called an ignorance prior. Anyway!

You mean, if naturalism and evolution are true, and there is a specific belief that is neither adaptive nor maladaptive, we should not trust that belief and reduce its probability back to our ignorance prior. That would be true if our beliefs were the product of evolution, passed down genetically. It's even true of cultural beliefs passed down between generations with their origins lost in time and no way to test them. (Well, mostly.)

But humans aren't faced with that situation. (Which is why Plantinga's argument doesn't tend to go that way, at least not where I've seen it before.) Humans have a reasoning ability hooked up to senses. We have had those senses and that reasoning ability tested by evolution in the areas where they affect our survival. Unless there's something radically different about other situations we apply our reasoning ability and our senses to that makes them not applicable, we should be able to trust them.

Unless there's some sustained, consistent hallucination that just happens to lead to humans being good at surviving while having no true beliefs, or there's something about our reasoning ability that makes it work well only when it impacts our direct survivability, we have no reason to pay any attention to this argument. But if we discover one of those, we don't come to Plantinga's conclusion; instead we conclude that we have no reasoning skills worth a damn, and we may as well give up on science and maths.

Plantinga's solution is to invoke a deity capable of giving everyone perfect cognitive and sensory abilities. This solution is contradicted by our studies into cognitive biases.