Referencing my guide, the ETs who have only appeared once in the ET Arcade are: The Forger, The Sensation, The Ex-Dictator, The Chef, and The Warlord. In other words, 14.3% of the ETs in the Arcade only appeared once.
I think all they’d need to do to fix The Fixer is remove the instafail conditions and just let you kill the target and retrieve the diamonds whenever you want. In the ET Arcade they removed the conditions for The Chameleon where you needed to kill him in a non-explosion accident and they didn’t do anything to change the briefing video.