Say you have an individual in the Army, likely an Appointee, that gets pulled to fill in as the Under Secretary of the Army due to an immediate vacancy. If that fill-in does a bang up job, and gets the nod to ultimately fill that roll through an appointment, then he/she must go back to the role previously filled until final confirmation. So, the vacancy from where the acting came from will remain vacant until they either step down, or return. Has nothing to do with the process Trump intends on using as it's just the process.^this is excellent analysis
I have no experience in this area, so I have no idea what the rules are. But rules as cited, I think the way to do it is to appoint Gaetz as deputy director and have him stand in as the director... since there isn't one.
As for the last part, I can see two main reasons why Trump is pushing so hard for Gaetz. 1)"because I can 2)it's part of some big behind the scenes thing that we don't have a full picture about at the moment. Serving as a loyalty litmus test, as some have suggested, may indeed be the