We've seen this sort of thing time and time again in Rock history. Once a beloved former member starts making special appearances with the band, a full reunion often becomes inevitable. Seeing MP gradually appear with the band members more often, attend Dream Theater concerts, and participate in side projects makes me feel like things are eventually going to head in that direction. Even MP's involvement with the Lost Not Forgotten series is a tell, being in a position where he's able to participate on the business side of DT again is not something that would've been imaginable a decade ago. I agree that if MP actually appears on stage with DT even to share the stage with MM, it's not going to be long before he is back in the band. I don't think MM would be dismissed necessarily, but I could see them reaching a mutual agreement to part ways. Again, I wouldn't be surprised if before long Mangini decides he's had enough of the touring life. He's made that decision before.
I don't think JLB's vocals would prevent MP from returning. He complained about JLB in and out of the band but it has never been publicly revealed as a factor in his departure. He clearly wants to play in DT again and if it really bothers him he can ask the sound guy to take JLB out of his monitor mix (which I think he might have already been doing).
IMO the biggest roadblock to MP returning is actually MP himself. I don't know if he is going to be welcomed back into the band in the same capacity as when he left, making creative decisions and being the public 'face' of the group. I wonder if he would rejoin the band without assuming a leadership role. I also wonder about his drumming and ability to play songs from the Mangini era. The more albums they make with Mangini, the more difficult it's going to be to avoid playing songs from those albums with MP. I guess they've already started to avoid that era themselves - we're starting to see a pattern of MM era songs being retired immediately after their tour debuts, but there are a few songs (mainly from ADTOE) that have become staples. Portnoy himself has said that Mangini is a better drummer and he would probably have to rework a lot of the parts, not sure if he wants to come into DT as a comparative weak link.
Overall though the fact that we're talking about this in context of MM's status in the band more than anything else makes me feel like a reunion is more realistic now than it has ever been previously. There was a pretty long stretch where he wasn't even on speaking terms with a lot of the band. That is a much larger hurdle to overcome.