I have a theory.
I believe Ford may really be at rehab (but just barely), and I think that's exactly what he wants (subconsciously; if he actually *has* a subconscious).
Unlike some, I don't think Rob is a sociopath. He's not "together" enough. He's got impulse control issues, but he also makes statements of regret or sadness when drunk. He's suffering from depression and rehab is a way out for him.
His brother and family want him in a short rehab. Not long enough to forfeit the mayorship, but enough to shore up the sympathy votes.
Ford hates his job. The disdain is visible in his eyes every time he has to go to a ribbon cutting and his work attendance is dismal. He even let keystone campaign promises sit in limbo (see: Sheppard Subway, etc.). The campaign isn't exactly going swimmingly and his support is a fraction of what it once was, as the launch turnout shows. But he's there, and running again because the family deemed it so. However, Rob, like many others with depression and guilt, is prone to self-sabotage.
So, my thinking is that he's going to consciously or subconsciously (and very publicly) sabotage his rehab. He stays in rehab for 90 days, he's no longer mayor. More than 6 months, and he's missed the election and has to wait four more years.
He gets out of having to run for mayor again, without the personal shame of losing an election that he already knows is probably lost anyhow. It's hard for his family to push him into running again if he's off the wagon in public. It's everything he really wants. He gets to take his ball and go home, in a bizarre way. Sure, Doug and Diane might be mad at him for a while, but after all the stuff Kathy's done she's still part of the family, right?