The FBI, the DoJ, the CIA, Rosenstein, Comey, and OSC/Mueller KNOW FULL WELL that Mifsud is a western intelligence asset, and that his contacts with the Russians are part of his role.
The whole narrative blows up when it is shown that Mifsud acted at the direction of the coup plotters. There is no way that Mueller et al want him to testify, or for his full associations to come out. These people are critical fact witnesses for the small parts that made up the whole. Each knows who directed them.
I find it possible, even very likely, that Mifsud, Halper, Christian Cantor, et al., were not read into the big picture of the coup plot, and were under the convenient belief that they were following orders in a legit operation.
The point of the operation was to fraudulently obtain FISA warrants. No wonder so many people are worried about this.