Yes on the first bit.
Yes on the second bit, although it could work with two controllers as well, provided you map the controllers to the right decks AND set the sound output options in the software correctly (say you have an SB and an SX, you’d map the SB decks to 3 and 4, the SX decks to 1 and 2 and ALL sound outputs to the corresponding channels on the SX).
Third option works also, you’d run the SB output into one of the two extra channels on the SX (which efficively is your mixer then). Mind you, you’d have to do FX on the SB.
It also depends on if you are using both controllers with their own laptops.