When you schedule a facebook live they start a "waiting room" that will switch to your live video whenever you start, so if you're a few minutes late it's ok.
We have the same stream key problem as you though. Typically we're just going live from a phone, and you can't schedule that in advance. My understanding is the scheduling only works if you are using a more professional set up with camera/computer/software. Please correct me if I'm wrong, more experienced streaming folks!