This may be a case where I just got it wrong, but to me it never made sense.
If Dany dies and Hizdahr is king, the Shavepate is a dead man and his entire revolution is destroyed. If his objective was to do a false flag operation, why not do it in a way that implicates Hizdahr – say, by having some of his Brazen Beasts attack Dany in the name of the Sons of the Harpy and Hizdahr King? Why not do it BEFORE Hizdahr marries her and gets political legitimacy? Hell, if he wanted to get rid of Hizdahr, why not have Hizdahr assassinated and blame that on the Sons of the Harpy? That would solve his political problem much more directly.
Look at the chain of events that happened in OTL – you have a poison intended for Dany, that only gets revealed because she brings Belwas with her and Belwas goes hog-wild on them before she even touches them, then the dragon shows up and Dany leaves, then Hizdahr replaces Dany’s entire team including the Shavepate, then the Shavepate has to sell Ser Barristan on a coup. Ridiculously circuitous doesn’t begin to describe it.
So it’ll take some explaining in TWOW if that’s what happened.
Have you considered that the Shavepate was trying to poison Hizdahr? Dany has never shown a particular fondness for Meereenese cuisine, so it’s possible he was banking on her not liking honeyed locusts. Strong Belwas merely got to them before Hizdahr did.
1. That’s a really circuitous way to kill him. Hizdahr never eats one of them, which is one of the things that leads Ser Barristan to suspect him, so he couldn’t have liked them that much. Why not poison Hizdahr’s personal drink, and thus avoid the risk that someone else gets poisoned first?
2. Why use poison at all? Poison is an incredibly chancy way of killing someone – you run the risk that other people get poisoned inadvertently, you run the risk that the person in question never consumes the poison by happenstance, there are antidotes and purges that can counter-act the poison’s effects, etc. Why not have some of his Brazen Beasts stab Hizdahr in the name of the Sons of the Harpy?
3. If the Shavepate wanted Hizdahr dead, why wait that long? Waiting until after he’s married to Dany gives his enemy political power and influence that the Shavepate doesn’t want him to have.
I think that the very fact that poison was used the way it was – i.e. in a general dish, as opposed to the intended victim’s personal meal/drink – points to the fact that there was no specific intended victim. If someone was really trying to poison Dany, they wouldn’t be stupid enough to put the poison in a dish that she was unlikely to eat, or at least eat no more than a bite or two of. Similarly, if someone was really trying to kill Hizdahr, they would put the poison in something he was guaranteed to eat all by himself, not offer to other people (including Dany). That leads me to believe that the poison wasn’t intended to kill anyone in particular, but rather was a general weapon aimed to cause chaos and sow distrust in the Meereenese court. If Dany ate the locusts and died (or even got very sick), she/her followers would automatically pin the blame on Hizdahr, and the two sides would proceed to annihilate each other. Same effect if Hizdahr ate the locusts and died/got sick. Same effect if the followers of either of the two ate the locusts (as Strong Belwas actually did). No matter who exactly got poisoned, the two parties would be at each others’ throats, and the fragile peace of Meereen broken.
That’s why it makes most sense that the poisoner was the Shavepate. He is essentially the same kind of schemer as Littlefinger – the man who got Lysa to send a letter to the Starks implicating the Lannisters in Jon Arryn’s murder; who falsely told Catelyn that the dagger sent to kill Bran was Tyrion’s; who went out of his way to sow distrust between the Starks and Lannisters in order to foment a civil war between the two families, so that he could emerge the winner once the two sides had finished each other off. The Shavepate is playing the same game in Meereen.
Addressed this elsewhere, but I’ll rephrase here:
Assassinating people randomly and hoping it leads to a situation in which you’re in charge is a terrible plan – and it’s kind of the the opposite of what LF did.
Littlefinger killed very carefully chosen people, used their deaths to the maximum extent possible, and carefully built up favors to get where he is, but even then it was difficult. Look at all the steps that Littlefinger has to take – killing Jon Arryn, sending the message to Winterfell, reinforcing that message to Catelyn, carefully steering Ned’s investigation, betraying Ned, arranging for Ned’s execution, arranging for the Purple Wedding, and so on and so forth. Yes, there’s an element of improvisation at times, but even at his most improvisational, LF never randomly kills or half-asses an assassination.
But using the locusts as “a general weapon aimed to cause chaos and sow distrust in the Meereenese court” is a terrible idea. If Dany dies or is sick, Hizdahr gains power and Shavepate doesn’t want that. As we see, there’s nothing automatic about about her followers going after Hizdahr – it takes a hell of a lot of prodding to get to there. If Hizdahr dies or is sick, that’s good for Shavepate, but if that’s his objective, why not be more pro-active about it?
The Shavepate is not dealing with so simple a situation that he can just wait to take over – he’s got the Yunkish army and the Volantenes who want to sack the city and murder him, for one thing.