The implications you're making here are more than a little disingenuous. The reason there are four Lyns versus three of Chrom and Xander is because people voted for her. With that in mind, a more honest claim would be Lyn getting her second seasonal versus Chrom and Xander getting their second seasonals. Is there a degree of hypocrisy here? Perhaps, but there are a few key differences still.
Chrom and Xander got their second seasonals during a time where seasonal banners were almost exclusively from Awakening and Fates. A good few people had the sentiment of "if you're going to keep doing seasonals from just those two games, could you at least have some more variety in which characters you pick from those games?"
Lyn's Second seasonal came out in the first seasonal banner to not include anyone from Awakening or Fates, and so a portion of people were just happy about that. It also came right after the Happy New Years Banner, which had two characters getting a second seasonal (Azura and Camilla). It was still a step in the right direction, even if you think someone else should have gotten her spot.
As far as Lyn getting a Legendary version, I firmly believe she should. Due to having a second seasonal in a still active banner, I do think she should wait a bit though. However I believe that as things are now, each of the lords should be given a Legendary form, and Lyn's second seasonal version should be no more disqualifying than Chrom's second seasonal version. Plus she's one of only a few options amongst the lords for a colorless Legendary.