IMO that is nonsense. Even though DA is facing federal charges, he is still facing the rape charges. If your argument is correct then they would have dropped those as well, but they didn't. Also, they were well aware that MP was already in custody on the federal charges when they initially charged him in connection with the mythical video. So, if the federal charges were going to take precedent preventing them from continuing, they would have known that then, it would not have suddenly been discovered now (unless they are totally incompetent). It is pretty obvious that these minor charges they have been making have been tactical, with little chance of actually resulting in a conviction. They know they wont get a conviction, but what these charges do is allow the prosecution to form an opinion of what happened in the case in the general public's mind without actually releasing actual information. That is valuable when it comes to trial, especially if their evidence is weak, because by then it may be common belief in the community that the accused are guilty, and it becomes much easier to get a conviction on the main charges.
The charges against MP and DA that were dropped were probably dropped because of lack of evidence, no matter what kind of spin the district attorney would like to put on it. They most likely withdrew the charges because they knew that those particular ones were going to be dismissed quickly, and a dismissal would challenge the credibility of the other allegations they were making. It is just strategic positioning so that they can go into a trial with the public conditioned to believing that the accused are probably guilty before the trial starts.
When the main case does go to trial, the testimony of people like D and S are going to be critical, any physical evidence they have collected is more likely to be corroborative in nature rather than definitive. It is the allegations of D and S that connect the other two to the case, so if D and S are impeachable due to physical evidence not supporting what they are saying, the trial is probably going to result in a dismissal. The fact that they are charging, or threatening to charge, these guys AFTER they have cooperated tells me that whatever they were telling LE was not being corroborated by whatever physical evidence they were collecting or trying to collect. That means that their case is probably very weak at the moment, not very strong. The new charges (and earlier threats to revoke immunity) are an attempt to intimidate these guys into coming up with an account that can somehow be corroborated, but without saying so explicitly since that would constitute witness tampering. IMO it is a sort of "wink wink nudge nudge .....say what we want you to say or go to jail" sort of thing.
My guess is that the new charges against DA probably came about during this process where DA was trying to figure out what he could tell them that would be what they wanted (they can't tell him directly what they want him to say - he has to come up with it himself). So he would have been telling them a bunch of stuff, true or not, but some of which ended up implicating him rather than the other two.
