Doing the new event on day 4 I went from quest 15 to 55 and still unable to maintain top 3. I did over half the content in one day and it wasn't enough. The leaderboard is too aggressive 

Agreed.  I have settled for trying to maintain silver 4-20 each day. Spread all content out across a couple days.  There's no way to beat the bots without spending gems and/or real money.

I had a pop plan today to open the gold box. For this I have to be in the top 50. For the remaining 2.5 hours I stopped playing. In the morning, I discovered that I was not in the top 50. How could this happen?

Has anyone gotten any legendary upgraded to the third tier? By my calculations, you'd have to have all the blues fully upgraded (not too hard) AND both reds upgraded once- which means at least 4 of the red chests (if you're lucky enough to get two of each). I suppose this isn't impossible- maintain silver for 3 days and grind the fourth with the tokens given from the blue chest . . . BUT, you'd also need to research 4 combos for each. with enough p2p equipment (research upgrade, 3rd slot, etc.) this might not be impossible, but remember f2p can't even get one of these cards until day 3-4 because of the first paywall block . . . A level 21 legendary card would be a wonderful backup to Emma, the game is sorely lacking tanks right now (and it'd be even better because it could combo/ has better attack), but I don't think it's possible for me. Don't get me started on 4th tier, lol.