Well, there are a whole bunch of questions mixed into one. Regarding limiting only certain users to an update, this is the best way to do it: How to prevent or limit updates.
I can try to do this: adding "the file" to "this update" and release it, then before I build and release the "next update" I will remove "the file" from "this update" (from File&Folders form in wyBuild) and then build the "next update". If the update don't find "the file" while building, it shouldn't create problem. Am I correct?
Yes.