Arcane Shift

Control of a battlefield is not lost as long as the state is closed. This changes how Arcane Shift and similar cards work, as you retain control of the battlefield as long as there are items on the chain, meaning the banished unit can be replayed on the battlefield even outside of showdowns.

187.4.c. If a player has no Units at a Battlefield and the turn is in an Open state, they lose Control of that Battlefield in the following cleanup, unless there is a Combat or Showdown ongoing there.

CR 321: while a chain item is resolving, a cleanup can not occur.

CR 323.6: while the turn is not in an Open State, there is no loss of control.

During the resolution, the state is Closed. After the resolution, the unit is pending and the state is still Closed. The condition for losing control (CR 187.4.c) is never met, and as such, you’re allowed to play the unit at the BF you were controlling if you banished the only unit at that BF. This may happen both during a Showdown and in a non-Showdown scenario.

Q. You play <Arcane Shift> on <Thousand-Tailed Watcher>. Can you use <Accelerate> to have the unit enter ready? And do you have to pay the 1 energy 1 mind power cost?

Yes, but you pay the accelerate cost.

731.+
Accelerate is functionnaly short for "As you play me, you may pay [1] and 1 Power as an additional cost. If you do, I enter ready."

Accelerate is an Optional Additional Cost to be paid as a player plays the unit with the ability.

350.+ Playng cards

• If an ability or instruction allows you to "ignore" on or more of a card's costs, set the appropriate Base Cost(s) of the card to zero.
• Apply aditional costs in any order (including Accelerate)
• Apply cost increases.
• Apply discounts in any order.