We finally, months later, received an email from Regent saying in essence 'airlines suck, and that's just the way things are.' Regent took zero responsibility for the situation or for leaving us hanging. They refused to reimburse my mom for the Uber ride she had to use to get back home from the alternate airport. So it was just a troubling situation all around.
I am just SO SO grateful Air Canada invested the time and patience to find us alternate flights home. Otherwise we would have been stranded in Istanbul and required to arrange fresh, brand new flights using our own money, which probably couldn't have been same-day, meaning we'd have to find a new hotel, etc. etc.
Even now, when I go back and reread my notes, I get very stressed. I am so grateful we got home and that Air Canada's person calmly got us through it. And part of what makes it awful is that it was WHOLLY unnecessary. The seats were empty. It's not like Turkish Air double-booked the seats. They were wholly empty and waiting for us. Turkish Air simply refused to lift a finger to figure out their tech snafu. And they were nastily hostile about it.