Imagine if they didn't have the knowledge of the gyroscope and stay stuck for enough time to be too late to save the world, then Mundy's arm is found in the ship giving the middle finger for anyone, futures Mundy would easily realize that is arms travel through time and not between universes. See, it's the reaction of past was being done in his present time, now aware that maybe his arm is in the past, he uses his remaining time to give all the essential data he gathered since he has no time to save himself. Change the past by sending info cross time would save everyone, so he desperately crawled until being found where he could give the message to save the two universes in the past, since he got stuck looking for the gyroscope for so much time, this was the only info about his time he could provide.
About the wall that for me is simple at least when two beings inhabit the same universe, the universe fix itself by killing both sometimes, so... as I saw many deaths were caused by a chain of events that for me means the universe fixing itself, the only excuse for me is Volkov (in my theory) he shares the body of the B universe into one luring him to attack the saboteur of his reality thinking was the same due to brain merge, as I saw everything on the ship was teleported into him, the worms, the gyroscope, and tragically parts of him of B universe, since he look's like to be talking to someone, I think it's his consciousness of B universe aware of a traitor, but confuse enough to not being aware of his own death. If got any question please say I will try check the post after some time. I think was not clear so I will put in a few words, resume about Mundy was (A past universe) and (B future universe) arms, and not dimensional.