If you wish to cast the revealed card without paying its mana cost, you must cast it while Yennett's triggered ability is resolving
This kind of grindy, variance-heavy self-mill plan isnt going to be as explosive or consistent as some other popular Commander decks, but I know there is a set of players out there who love to play a methodical and reactive style, and Winter lays out a really cool way to do that
After declaring attackers, you can impulse draw with cards like March of Reckless Joy, build up mana for a card like Leyline Tyrant, or use combat tricks like Lunar Frenzy
Grievous Wound's last ability triggers and resolves after the damage is dealt
This is an incredibly powerful effect, made better since Timetwister doesnt exile itself
Overall) - Regal Force This is here for that ETB trigger to draw equal to your Green creature count