They don't have to win the season, they just have to appear in the #1 spot and have a screenshot posted in the comments here.

After someone's name resolves yes, they can be added again after the current season ends.

Feel free to add someone even if they aren't in masters currently, but please don't add anyone just to bet them down and profit from your own liquidity.

Edit on Shenanigans:

Moving a market's price where you have a large position in order to get ahead in leagues has always been allowed, and so would also count here. Buying up shares that someone else sold indiscriminately like the Mira incident would also count, as long as I don't think it was a deliberate profit transfer to a specific person.

Most other shenanigans wouldn't count though. Getting a bunch of league points from a market that should be unranked before it is unranked wouldn't count. Likewise, I wouldn't count if someone got to #1 just by betting on a market that is designed to N/A. I also won't count any glitches.