If Yes Man isn't on the battlefield as its first ability resolves, or if the target opponent is an illegal target as the ability tries to resolve, nothing will happen
Instead, players can crack into Play Boosters and Collector Boosters of Standard-Legal cards, whether bought as individual packs, as part of a display box, bundle, prerelease box, or as part of the particularly collector-friendly Spidey's Spectacular Showdown Scene Box
Treasure Vault might also be a fun addition to this deck if players are having trouble getting the lands they need, but it shouldn't be too much of an issue with the flip lands in this list
staple status ensures 40% ROI, especially in full-art variants
So, attacks made with both sides of Kefka will count towards the 21 damage needed to take someone out of the game with commander damage