A lot of assumptions about the circumstances surrounding Mike Mangini. I wouldn't bet on anyone's particular hunches being true, since we really don't know. It may have been a one-way decision where the guys in DT decided to move on and bring Mike Portnoy back in.
But for all we know, it may have been the complete opposite, and may have been something initiated by Mangini where he decided to step away for whatever reason (wanted to go back to teaching, got another opportunity, decided that the itch to be in a full-fledged writing and touring band had now been sufficiently scratched and he was now ready to do something else, or what have you). It could have been a mutual decision. It could be any number of things. We just don't know. The only thing we do know is that everyone involved kept these secret locked down until the agreed time for it to be released. And what I can glean from that is that any comments from anyone in the last few months about DT's plans would have been calculated to avoid even a hint of MP returning, so I don't really trust trying to read anything else into anything that may have been said fairly recently. The band didn't want this to leak, and Mangini being the consummate professional he is wouldn't have jeopardized that by making comments that could fuel any sort of speculation about him leaving, even if he knew full well what was going on and even if, hypothetically, it was 100% his decision.
Along similar lines, it doesn't surprise me that Sheehan didn't know either. Again, they kept this information very tightly buttoned up and did not want it to leak. The ONLY way to do that is to keep strict limits on who knows what. Telling anyone outside of the agreed-upon circle means loosening the control over a potential leak, and that would include musicians in other bands outside of the six involved, no matter how close they might be to any individual band member. I don't know Billy, so I'm not suggesting Billy might do this, but it isn't hard to imagine a situation where, for example, Mike tells Billy last night and swears him to secrecy, but Billy is laying there alone later that night in his bunk on the tour bus, and says, "well, the press release is only a few hours away now, and most people are asleep. Surely there's no harm in telling [closest best friend in the world], and swearing him/her to secrecy." And that person has a similar reaction and tells his/her best friend, while swearing him/her to secrecy, and so on. They kept a lid on this and did a good job. It does not surprise me one bit that Billy didn't know ahead of time even though they are touring together.