No one in particular wrote:TL;DR - my interpretation is that Date-a-mancy is focused on understanding how things work through interaction. People on dates (Uhura), units in fights (Laurel of Napster) or Rules colliding (Parson's Foodfight tactic). Parson may not be very good at the person to person level, but he always was a big picture thinker.
Also, we've seen casters use scrolls outside their disciplines before (Gobwin Knob's healing scrolls, the "Summon Perfect Warlord" spell, Wanda improving the tower with a Dirtamancy scroll in IPTSF 20).
We have seen experienced casters use scrolls outside their discipline when they have some experience casting in that discipline before. Wanda has experience and skill with at least the theory of several disciplines, so she can cast scrolls with it, either way. She even comments while casting the Perfect Warlord spell about how hard it is because she isn't a Findamancer, though earlier discussion with Sizemore revealed that she is quite good at a number of magics outside croakamancy.
I would say that (for me) the primary reason that I think Parson is a Carnymancer is in the way he thinks. He doesn't just think "big picture" or see the interactions. He actually revels in beating the no-win scenario by bending and breaking the rules...to the point that the activity he was engaged in while being summoned was creating a game scenario that forced the players to seek a way to break the rules somehow. That was actually his stated intent.
He questions the rules. He seeks ways to get around them. He looks for the ways to take actions that work in the rules without being popularly done. He looks for advantages in the rules that seem like they should be against the rules but somehow aren't.
Either way, I don't know how strong the evidence that he can use the scroll points towards any discipline at all. And, as it was pointed out above, Parson may not be a caster of a specific discipline...he might be a caster for all disciplines. He isn't even aware of his own stats, so it's hard to say what he is and isn't capable of.
(By the way, I'd totally bet against you, No One In Particular, if I thought the bet would be resolved sometime in the next 12 months. But, I honestly don't think it will. One piece of evidence to me is the way Parson interacts with the Archons...really not Date-a-mancer material.)