Help understanding the wiring of this switch
I think you're correct that either the upper right or the bottom are the line in. If you disconnect at the nut and probe, you'd be able to figure out which is hot. (Not that it really matters... you simply need to ensure that they're connected when you re-do everything.) Be safe when testing energized circuits.

You should consider a more elegant power feed to the existing switch by bringing pigtails off the nutted-together blacks. (And it's better to not use the backstab in switches, as they did on yours.)

Is this-all supposed to go back into the existing 1 gang box with a double switch? I haven't done the calculations, but my gut says you'll exceed the fill rules. I suspect a 2 gang box would be fine, but do the math.