In an earlier TOTD, I wondered why the Bureau doesn't seem to trust Bracken to perform in the DiF role to which he'd been assigned (under some pressure of time) at the beginning of Scorpion. Indeed, I noted, there's a point at which Bracken actually has his authority usurped by Croder's unexpected arrival to take over management of the operation.
There may be a reason for not letting Bracken run the show, and it's given by Quiller, in the form of one of those endless rumors he's heard batted about the Bureau:
"I didn't know much about [Bracken], only a few things I'd heard over cups of tea in the Caff between missions; someone had said he'd been thrown out of an instructor's job at Norfolk because he'd used a live charge to demonstrate his de-arming techniques, and someone else had told me he'd murdered his mistress and been acquitted because the Bureau had suppressed some of the evidence; I didn't necessarily believe either story but the truth was probably somewhere there in the background." [4:53]
Now, if either of these stories were true in whole, or even in part, that'd be reason enough to disqualify Bracken for a mission as ultrasensitive as preventing Brezhnev from being blown sky-high. It's worth noting that two things important to bringing Scorpion home (cold-blooded murder and bombs) are dark chapters in the rumors about Bracken's past. Thus, while Croder might have been comfortable letting Bracken handle the admittedly difficult access and lock-on phases of Scorpion, he would've been justified in assuming that bombs and murder would be just too much for Bracken to handle. As a side note, it wouldn't be too much to assume also that Bracken's being sent into the mission might be an expression of the Bureau's absolute control over *yet another* of its minions who's committed murder and had to be kept from discovery.
A reasonable scenario: Bracken was shot into Moscow because he "knew" the place, but was regarded from the outset as a stopgap DiF, someone Croder had to get in as an element of a mission he had to pretty much throw together before getting into signals with Q from Geneva. That would explain a lot about this complicated mission.