Uh, DC time is so freaked, it's not even funny. But, let's try to put some stuff in perspective.
Regarding Batman and Robin and their respective ages, I don't even know where to start. I would say that if Bruce started being Batman when he was 20-21. And Tim started being Robin when he was 13-14 and continued being Robin for eight years, that would put him leaving to be Nightwing around the age of 21-22. Not entirely unplausible. In pre-crisis time, it's around the same time that Wally West gave up being Kid Flash. Of which, Waid's Flash series would put Wally in his early to mid 20's, continuing until his early 30's.
The 1940's will always be Pre-Crisis stuff. Up until Crisis on Infinite Earths, it was strongly set in continuity that everything done during WWII was done by a Batman on an alternate Earth. This would allow those stories to remain within the DCU, while showing Batman still maintaining his Playboy image forty years later. If we use this theory, then we can surmise that the two batman's in the Plastic Man pictures are completely different Batmans.
Now, after COIE, Batman's continuity was utterly ripped to shreds. But, we won't go too detailed with that. Marv Wolfman once gave the best description of pre-crisis vs. post-crisis Batman. He said that, essentially, any pre-crisis story could have happened to post-crisis Batman, as long as its not directly contradicted by post-crisis events. For example, the stories in the 40's could have happened, provided they aren't date specific, because post-crisis Batman started fighting crime in the mid 80's. Batman may have killed criminals in the 1940's, but that's not continuity, because post-crisis Batman doesn't kill. Ever.
Regarding the future, I think it's all plausible. Now, I haven't read much Kamandi, so I could be wrong. But, isn't the date of the Great Disaster relatively unknown. Is it possible that it takes place after the 25th Century, maintaining that piece of continuity and keeping Legion safe as well?
According to the Kamandi Wiki, the Great Disaster was always seen as one of the possible ends for Earth 1. Since the COIE, he's supposedly not in continuity. Which is too bad. I think it's one of the more interesting ideas within the DCU.
