A release date has not been confirmed. It could be this Tuesday, the 16th, for all we know.
I feel as though a partner for Ada's campaign isn't necessary at all for Professional, or any difficulty level for that matter.
Her campaign is easy enough to complete on Professional without any struggle, assuming you've completed it on Normal before your Professional run. This way, you know the layout of the 5 chapters and how to handle each encounter within.
Currently Playing: Battlefield 4 (Xbox One) | Skyrim (Xbox 360)
Upcoming: Zoo Tycoon & Halo: Spartan Assault