If you do, you do so as part of the resolution of Jetsam
Path to Exile is the optimal removal for the deck (especially since we can sometimes eliminate the downside of giving our opponent a land with Leonin Arbiter), but Declaration in Stone is a fine budget replacement with some upside against token decks since it exiles all copies of a creature
Slots are registered in the platform only after payment has been received
You can, however, pay any additional costs
In Pioneer, you have both the Theors block and Theros: Beyond Death to play with for constellation, while Kaldheim gave us an easy way to make lots of enchantment tokens with Niko Aris
Five-mana black spells in Modern are reserved for game-winning effects like Ad Nauseam , and this is not going to have that kind of impact unless your opponent is sitting with no creatures, no hand, and has three life left