A combination of power in the game, rarity and popularity as a Pokemon are what has driven the price of Rayquaza GX rainbow rare
A lot of the cards in the Rogues deck like Drown in the Loch also benefit from opponents having lots of cards in their graveyard, so the Battle forcing each player to mill three cards is a nice bonus
You choose modes and targets as normal
Yenna, Redtooth Regent Yenna, Redtooth Regent can generate a lot of constellation triggers for things like Archon of Sun's Grace and can ramp pretty nicely with cards like Wild Growth, but is that better than running a more efficient Selesnya commander with a card draw ability
As for what could be in the missing spots, 439 is missing, which is a white card between Dion and Minwu, 457 is likely a red card before Clive alphabetically, a multicolour card at 485 between The Emperor of Palamecia and Garland, 504 between Rufus and Sephiroth, and 515 between Vivi and Xande