So if it wasn’t the Shavepate who poisoned the locusts, any guesses as to who the culprit really is? The points you made about it being a sloppy attempt if either Dany or Hizdahr were the target makes me think of the catspaw Joffrey sent to kill Bran. In that case, a sloppy assasination attempt indicates a not-so-bright culprit. Could it be the same with whoever poisoned the locusts? Or did they have some other purpose besides killing Dany(or Hizdahr)?

So I would start from the question: who benefits if the assassination attempt had gone as planned? Let’s say that Belwas doesn’t come to the fighting pit that day for whatever reason, and Drogon doesn’t unexpectedly show up in the middle of Daznak’s Pit. 

Now, given the sloppiness of the method, it’s not clear whether Dany would have died from eating a few locusts or whether she just would have been made badly ill, but in either case the result is that Dany is out of commission (whether temporarily or permanently), which leaves Hizdahr running Meereen. And we know what happens in OTL when that happens: Hizdahr takes the Great Palace, installs his cousin as commander of the Brazen Beasts, tries but fails to take command of the Unsullied, and tries to placate the Yunkish to the point of considering killing Dany’s dragons. The main difference between OTL and our new timeline is that there would be much less confusion about where Dany was and whether she’s coming back, as she’d either be dead, dying, or “confined to her sickbed” or the like. 

Overall, this follows the interests of the pro-slavery faction in Meereen – although notably there’s a bit of a difference between them and the Yunkish army outside, since while the Great Masters want Dany gone and slavery restored, they don’t want their city sacked, whereas the mercenaries want to loot the city and the Yunkish have promised loot to the Volantenes and others. So we should look for suspects to members of that faction:

• Hizdahr is plausible. He’s the one who arranged the box, he’s the one who offers the poisoned locusts, he’s the one who benefits the most materially from Dany’s sidelining. On the other hand, he’s the most obvious candidate, and he’s the most implicated in all of it. Personally, I lean towards him being a catspaw or patsy rather than the overall planner. 
• The Harpy is also plausible. Now that Hizdahr is married and in line to take over with Dany gone, the Sons of the Harpy have a strong motive to complete their work by getting Dany out of the way and having Hizdahr carry out the counter-revolution. To me, the main counter-argument is that the Harpy tend to be a lot more stabby with their assassinations. OTOH, if the point is to get Dany out of the way subtly, so that Hizdahr can take over without opposition, I could see it. 
• Reznak, assuming he’s not the Harpy, is an outside possibility. There is the whole “beware the perfumed seneschal” prophecy, he’s definitely part of the pro-slavery faction, although definitely more on the collaborationist end than the militant end. He was in the box, close enough to whisper in Dany’s ear, and could have poisoned the locusts…although if he was that close to Dany, why not poison her wine? 
• Galazza Galare only really works if you think she’s the Harpy. She was all the way across Daznak’s Pit from Dany at the time, so a good deal of prior organization would have been needed for her to mastermind the assassination attempt. OTOH, if she is the Harpy, she’s way more subtle than we often think of the Sons as being, so her being in a perfect position to observe Dany without being suspected is quite interesting.