Can the Morgan–PIXY be used in Campaign Mode immediately?
No. A Premium Ace Pass Plus voucher can unlock it immediately online, but Campaign Mode use requires completion of all campaign missions.
Confirmed explanation
The mode restriction is stated in Bandai Namco’s official FAQ. Immediate voucher access should therefore not be described as a campaign shortcut.
What is not confirmed yet
The exact post-clear menu, New Game Plus behavior and later acquisition route are not documented.