Having to do a major re-work on this adventure because the scenes from the party at the first part immediately caused my PCs to suspect pursue Ensor, then turn around and interfere with/stop the assassination. This means I now need to re-do the rest of the adventure to the extent that solving an "attempted assassination" doesn't quite have the same drive and urgency as having a fresh body on the floor.

Any one else run into this?

Well, kill him in a different way. I honestly have not even red through that one, but I will as soon as I get a chance, probably tonight. But I have, in the past, had players stop an event that was supposed to happen. In this case it sounds like you have two options, have the players continue to hunt for the reason / man behind the continuing assassination attempts, so they have to split resources between finding out whats going on and keeping him alive. Or kill him when the players are not around and just change some minor facts to account for this minor change in time line.