RE: .204 Ruger for small varmints
About the only feature of the .22 calibers versus the .204 is that they .22's are somewhat better bucking the wind. However, the .204 has proven extremely accurate and at the velocities it achieves, it is very destructive, so it will definitely take care of all the critters you plan on using it for, including coyotes.
It is not quieter than the .223, but perhaps a bit less noisy than the .22/250 or .220 Swift. Of course, at the present time it is cheaper to shoot the .223, due to the availability of military ammo and components. I suspect that this situation will remain the way it is today.