Originally posted by Ken Arromdee:
If the Legion wanted to, they could travel back to the 21st century, free Mon-El, then go 990 or so years in the future to just before they were supposed to free him, and put him back. Then tell him to pretend he was in the zone for 1000 years. This wouldn't change history much at all.
They could even mess with his memory, so that he didn't have to pretend.
And it doesn't have to be the Legion. Pretty much any other group with access to Zone technology could do this. Mon-El could end up working with the R.E.B.E.L.S. for a couple of decades, so long as his adventures with them didn't make it into the history books, and it wouldn't affect things at all.
His 'thousand years in the Zone' is carte blanche for him to have a thousand years of adventures that we've never heard of...
And it would make Superboy / the Legion look a lot less jerkish if they ended up with some knowledge of this, to explain why Kal-El waited a thousand years before doing anything about the situation.
A fun soft retcon might also be that the cure for lead poisoning required a long time to work, that Brainiac 5 introduced it to Mon-El back in the recent 21st century adventures, and that it has spent ten centuries 'brewing' in his body, and finally, by the 30th century, was strong enough that he could get a final booster and stay out of the Zone for good.
It's been previously established that a 'side-effect' of the cure was being able to function under a red sun. (Which is just hella crazy. The side-effects of real cures are like 'nausea, diarrhea, vomiting, thoughts of suicide...' not like 'oh, also removes your *other* greatest weakness!') Perhaps Brainy went that extra mile and made a 'cure' that reconfigured Mon-El's entire body on a genetic/cellular level so that he's no longer susceptible to lead *and* can metabolize red solar energy, which required a bit more work, since it was rebuilding his entire body cell-by-cell.