Recall, dominance by the OP's definition refers to a pokemon being able to win against a dominated pokemon regardless of what the opponent does. That is, if we are trying to test whether or not Mewtwo can dominate over Spiritomb, we have to assume that Mewtwo has no knowledge of what Spiritomb is going to do while Spiritomb knows what Mewtwo is going to do.
In other words, Me First will always fail... because Me First has 0 priority (i.e. it is NOT a priority move as is commonly mistaken), meaning if Spiritomb is using Shadow Sneak or Sucker Punch, Me First will always fail.
Trick Scarf Mewtwo will always lose to a Specs or Band Spiritomb.
Specs Mewtwo loses to all Spiritomb that have Sucker Punch.
Bulky Mewtwo without Taunt loses to Spiritomb with Taunt + Sucker Punch.
Bulky Mewtwo with Taunt loses to Life Orb Spiritomb with Shadow Ball / Dark Pulse / Shadow Sneak
Just some calcs:
Life Orb Quiet Spiritomb will 2HKO 252 HP 252 Sp Def Calm Mewtwo with Shadow Ball / Dark Pulse.
Life Orb Brave Spiritomb will 2HKO 252 HP 252 Def Bold Mewtwo with Sucker Punch.
With that said, if Mewtwo is without Taunt, Spiritomb can taunt after Mewtwo has used Barrier, Calm Mind, Bulk Up, or Amnesia on first turn.
If Mewtwo used Barrier or Bulk Up first turn, Quiet LO Spiritomb will 2HKO Mewtwo while Mewtwo fails to 2HKO back.
If Mewtwo used Calm Mind, Brave LO Spiritomb Sucker Punch will 2HKO while Mewtwo fails to OHKO back.
If Mewtwo has taunt, it will need Taunt, Recover, Calm Mind or Bulk Up, Attack. Regardless of the set, Spiritomb will 3HKO with Shadow Ball x 2 + Shadow sneak (i.e. Mewtwo will have to taunt first turn, and then calm Mind or Bulk up on 2nd turn).
Blah, More later if I have time.