2021-11-19 : If you cast a spell with X in its mana cost this way, you must choose 0 as the value for X.

2021-11-19 : 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).

2021-11-19 : 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.