If the target of Discipline isn't in play anymore upon resolution, does the player playing it still get the draw 1? And does it work the same for Stupify and Void Seaker?

Yes, the player who played Discipline still draws 1 card, even if the targeted unit is no longer in play when Discipline resolves.

Why this happens

According to RiftJudge FAQ #10008 and Rule 359.3.e, Riftbound spells resolve as much as possible even if their target is removed or becomes illegal while the spell is on the chain.

Sequence of Events:

1. Play Discipline: You play Discipline, targeting your unit. It is placed on the chain.
2. Opponent Reacts: Your opponent plays a card that removes your unit from the battlefield (or otherwise makes it an illegal target).
3. Chain Resolution (LIFO - Last-In, First-Out):
   • The opponent's removal effect resolves first, removing your unit from the board.
   • Discipline then resolves. Because the instruction to "Draw 1" is not dependent on the unit successfully receiving the +2 Might, it still executes. The instruction to give the unit +2 Might is ignored because the unit is no longer a valid target (per Rule 359.3.e.5).

As noted in RiftJudge FAQ #7790, spells always resolve as much as possible, and the draw instruction does not require information about the target to successfully resolve.

I noticed multiple unrelated questions. I've answered the first one — please ask the others in separate messages so I can give each one proper attention.