A change of behavior isn't nessacarily a bad thing, any tropical cold blooded animal at 66 degrees would adjust and slow down. Being at 66 degrees constantly for something like a white tree frog will stress them out and slow digestion, but a few days of cooler temps won't kill them or cause any lasting effects.

Aiming for 75-78 degrees sounds about right.

Good luck with the space heater! It worked fine for my toads when they where down in a basement for winter months. I've never used a human heating blanket, I'd check the instruction and warning labels though - I highly doubt they can be turned on 24/7. You might risk a fire. I most certinally wouldn't try to attach it directly to the tank.