Here comes my reasoning. We have only a few morsels of confirmed information and a whole lot of minor hints, so I'm going to keep a tally while rambling about this. But first, let's define the three competing theories and penalize those two that are less likely explanations.
1. This is a gathering of potentially pro-Viserys actors to hash out an agreement for a common goal. (What Jeanna was told is happening.) Points: 0
2. This gathering was arranged by another party for an unknown purpose. (As of yet unknown intrigue goal.) Points: -3
3. This is the setup for a magical ritual. (The way-out-there theory.) Points: -6
Let's first look at the composition of this gathering:
- We have Jarlar, who is politically influential and would stand to gain from our conquest of Lys if he aligned himself properly before that. Make that 1 point for Theory 1. However, this also means that he could have been invited as part of a anti-Viserys agenda, so it's also a point of Theory 2.
- Then there is Yango Reft, who we know little about, but he seems slightly inclined towards Viserys and might have some power behind the mask. Again, this could swing both ways, so 1 point for Theories 1 and 2.
- Then we got Nikeos Armatis, the lotus trader. He is vaguely aligned with the Spicers, which means he could be flipped to some other cause, but he isn't anyone that would fit in a pro-Viserys alliance. He is a brutal drug-kingpin and anyone knowing a bit about our rule would realize that once we take over Lys, we would round him up and make him spill his guts, first figuratively, then literally. He wouldn't be so powerful if he was an idiot, but that means he isn't dumb enough to not see that Viserys taking Lys would be the end for him, no matter what. Likewise, the organizer of this wouldn't have invited him for that very reason. So I'll award -1 point to Theory 1 and 1 points to Theory 2.
- Now it get's interesting, with Eferil Irjen, the rogue Red Priest. He isn't politically powerful and would be a distant second pick to the Flame Keeper of Lys if you wanted someone to represent the Reds in this. At the same time, he has some arcane interests, which sets him apart from the others on this list even more. He makes no sense if this was a pro-Viserys gathering, especially as we had widely known amicable contact with the Flame Keeper. So I'm giving this -1 for Theory 1 and 1 points each for Theory 2 and 3.
- Gorthor the Lecher, now that is an interesting character. Given his proclivities, he likely has quite a bit of power in the city in the form of blackmail material on other figures and he is the high priest of Trios in Lys. Again, it's not hard to guess that Viserys would consider the guy repulsive as fuck, but his connections would still make him valuable to invite and Gorthor himself would probably not notice that his behavior would be ill received. On the other hand, they didn't invite the Flame Keeper, so it's questionable why they wouldn't invite a lower ranking, but more acceptable priest in his stead if this was about the faith of Trios being represented. Overall, I'm inclined to give him points for both Theory 1 and 2.
- Now here comes the kicker. Jeanna Forly. She has no reason to be here, except as a mouthpiece for someone else maybe, yet she was on the original list of invitations. Why? She has neither power, connections or any useful talents like the suspected sorcerer, so why invite her? If the organizer of this wanted a mouthpiece, he could have invited someone with more value that was already aligned in the right direction. There is little obvious reason to pad the guest list with her. At the same time, her 'mission' is a farce that mostly seems meant to keep her busy. Agent A was in contact with Jarlar before this meeting, so after already directly approaching him, there is no reason for him to step back and try indirect methods next. Either Agent A has a large hand in all of this, or Jeanna was approached by someone else borrowing the identity of the suspected Westerosi agent. She makes no sense at all in a pro-Viserys gathering, so -2 points for Theory 1. At the same time though, she makes no sense as a guest under Theory 2 either, for she seems ill suited for any spy-work and her handler implied he had full shape-shifters at his disposal, so -1 points to that theory too. The only real explanation for her presence is some hidden property of her that makes her valuable for other purposes, thus making it more likely that she would be here, if this is a ritual setup. Weak evidence it might be, but evidence none the less, so 1 point to Theory 3.
Tally so far:
Theory 1: -1
Theory 2: 1
Theory 3: -4
So... by summing up all those little bits of evidence, it now seems more likely that there is something else going on then this being a pro-Viserys meeting.
Also, the way-out-there-theory is gaining some ground even before I got to the evidence about that.
Resuming this post in a bit, but throwing this out now so that people can start dissecting it if they want.
I completely missed that. 
