Hi Brett. I'm going to break up the answers by post.
"1 - when is Device Health likely to move out of public preview?"
In our current Public Preview state the solution is available to all OMS workspaces. We are integrating feedback and testing additional functionality. At some point soonish-ish (not sharing any dates today) we will declare "General Availability" for the v1 round of features, and then start previewing new features.
As a cloud service our expectation is to iterate continually, so expect ongoing Preview states for new features as they arrive.