Thanks for the questions and sorry for the delay (holiday stuff). I’m glad to hear you're enjoying the game and asking (pestering) your friends to play!
Question #1 - The card does NOT need to be playable before you draw the cards that have been discarded after the picked-up card. The card can become playable by way of the cards you have picked up in the Wilds.
Question #2 - You can do it that way! You could also roll them all at once. So, you could use all three uses of the Tall Grass gift and end up with 3d6 armor.
I hope that clears up everything. I’m always happy to answer questions like these so feel free to keep ‘em coming!