I don't know enough about the situation to have an opinion about much of this, but at least on this one I don't think you need grounds to disbelieve stated reasons for resignation. I've personally witnessed many people resigning and giving reasons like "to focus on my family" or "to focus on my health" or something when in reality they were parachuting out before getting fired or were resigning for other reasons but didn't want to burn a bridge by telling the truth. I wouldn't be surprised if being untruthful (or only partially truthful) in resignations is more the norm than is being honest, and when talking about politics that probably goes up even more.