Hi @johnlloyd_13,
If I understand correctly, you will have 2 deployments - one with primary and active in existing DC, and another you are moving to secondary DC. If this is the case, then two devices are behaving as two independent devices, and you'll need to have AAA settings on secondary device too. Depending on your configuration (using aaa-server or not), yes, you can still use local user account.
Regarding access to OOB address, this can be a bit tricky. Assuming that it is routable network even in another DC, if you plan to keep same addresses used currently, when new cluster comes up, and it figures out that it doesn't detect active mate (since no HA link is active), secondary device too will become active, meaning that it will take active mgmt IP (leaving you with 2 devices using same IP). Same would go for all interfaces (data plane), unless you control this (like shutdown neighbouring ports).I would think about this approach bit more before deciding to do it this way.
Admin context provides you possibility for remote management, so if you plan to use SSH or ASDM access, yes, you'll need to create user under admin context. Once in admin context remotelly, you can shift between contexts, so you don't really need another user (unless you want other contexts to be directly accessible). However, if on console, you are going to system execution space, and there you need to authenticate too. I can't really remember from the top of my head, but I believe enable password is enough here.
BR,
Milos
