Isn’t that a Navy-Marine Corps thing? Fleet Reserve?
Whether or not retirees can be prosecuted with UCMJ is in contention. There are a couple of cases the SCOTUS has declined and a couple more working up through the system. A District Court judge ruled it unConstitutional in 2019.
It wasn’t a common practice, it seems to be getting more so.
No, when you retire after 20+ you are transferred from active service to the retired reserve. You get orders reflecting that transfer and noting that as a member of the retired reserve you are subject to recall to active duty at the order of the Commander and Chief.
You only get the title by being the spouse of the president, The position isn’t open for application, it is a closed process, open only to a specific family member from the beginning.