19 Nov 2021
If you cast a spell without paying its mana cost, you may choose to pay additional costs (such as kicker) but not alternative costs (such as flashback).
19 Nov 2021
If you cast a spell with X in its mana cost this way, you must choose 0 as the value for X.
19 Nov 2021
If you choose to cast the spell, you must do so as Spectral Arcanist's ability is resolving. You can't wait and cast it later.