Just make sure it is relatively recent and the battery is alright. (Batteries will only last so many charge cycles.)
Not out of the box. There were adapters, but I think those were for old 30-pin iPhone connectors and I have never tried them. IMHO just stick to Bluetooth. Some trainers will aggregate Ant+ and Bluetooth signals, so you might also look into that. In the worst case, get a new heart rate strap. They aren’t too expensive. The current top recommendation is a Polar H10 or Polar H9. I got an H10 recently, and it has been rock solid.