I thought that the Shavepate article was the strongest of the meereenese blot articles, but I doubt that the Shavepate was the poisoner. He has less to gain than the Green Grace, the Shavepate's position is entirely dependant on Daenerys. If he did arrange to have the locusts poisoned then he had huge good luck in Daenerys not eating one and dying. He wouldn't have been able to guarantee his power in the event of her death. It remains highly suspicious that Hizdahr didn't eat the locusts (although of course it would have been possible for the Shavepate to have warned Hizdahr indirectly).
The peace in any case wasn't real because it was Volontis and Qorth that paid for the war not Yunkai and certainly not the sons of the Harpy. Daenerys managed to sell her principles down the river to gain time for her enemies to arrive, not her finest moment. On the other hand the sons of the Harpy played a weak hand and won within the time scale of ADWD, forcing Daenerys on to the defensive and getting a good deal of what they wanted. If the slave army that Volontis is sending was reliable they would stand a chance of getting slavery restored, as it is the long term outlook probably isn't too good for them.