I don’t know about your players, but mine never do as they’re expected to. You might wish to have some kind of contingency plan if the players decide to kill him off — for instance: Mishra doesn’t go himself to speak with the characters until he has secured the party’s cooperation. Maybe he sends a zombie with a telegram. Alternately, Mishra has a girlfriend/wife/son/daughter/retainer/butler who tries to secure the group’s cooperation first.
Just in case the party rejects the opportunity to help Mishra, have a backup plan. For instance, Mishra owes the Merchants Guild a lot of money, but since Mishra’s legal status is in question his money is tied up in court. If Mishra is declared “dead,” the money goes to his daughter by will so the Guild doesn’t get paid, but if Mishra is restored, they get their back pay. The Merchants Guild will hire the players to help straighten out their debtor so they can get their hands on the money. This would be an alternate way of making the players accomplish the same goal, so you could keep the Merchants’ Guild in reserve unless you actually needed them. (Of course, you could also throw them into the game in addition to Mishra himself for a little intrigue and a who’s-conning-who deception.)
There could be some problems if the group has a good cleric — I don’t remember that clerics get the option of deciding which undead they’ll turn and which will be spared. If you design this campaign, you might want to be careful how you handle party encounters with undead. The cleric might accidentally turn Mishra.