“So it is unusual, to say the least.”
Unusual, but not unheard of, not unprecedented, and not invalid. Thus, who resigned as pope before Pope Francis became pope has no bearing on the validity of Pope Francis’ pontificate. All that matters is that Pope Benedict validly resigned, and Francis was validly elected. That’s what happened. That’s the reality of things.
Which means that Francis is the valid pope of the Catholic church, and like it or not, that's the reality of things.
Therefore Catholics are prohibited by Unam Sanctum, from having the option of not being in submission to him.