18 Feb 2022
You choose which cards that you are playing from exile as you play them, not as March of Reckless Joy is resolving.
18 Feb 2022
The mana value of March of Reckless Joy while it's on the stack is the value chosen for X plus 1, no matter how much mana you actually paid to cast it.
18 Feb 2022
You may exile more cards than necessary for March of Reckless Joy's first ability, but you can't reduce the mana it costs to less than {R} this way.