Since the milling isn't a cost, I should still mill the last 2 cards of my library, right? I just don't get the -X/-X?

What about the last sentence in 118.12? "The 'If [a player] [does, doesn’t, or can’t]' clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred." If I milled 2, did I not "start to pay a mandatory cost"?

@user10478 In this scenario, NAP has priority: AP has cast Oblivion Ring and passed priority, and NAP is considering a Counterspell. So it's perfectly fine for them to propose a shortcut.

Note that if you connect with both Deveris, you will get four triggers, not just two. Each of them sees two creatures deal combat damage to a player, so each of them triggers twice. You can interleave activating the Prophet in between each of these untaps, as GendoIkari says.

@r.e.s. I see. I was thinking that the deck itself had to be emptied, not just the 9 columns. But in that case there's an even simpler proof of impossibility: you can't pick up 52 cards in sets of 3!

If it were a generational prank he would have told you to remove groups that add to 9, 18, or 27 instead. It's easy to prove this game can never be won. The deck's total (measured mod 9) never changes as a result of such removals. So it's impossible to get from any starting configuration (sum 340 or 380 depending how you count aces) to an empty configuration (sum 0).