You said the puddle was under the car. If the heater core was bad it would leak and reek of coolant
in the car.
The bottom line is, did the bypass fix anything?
The heater on my Sable was bypassed when I bought it. When troubleshooting I hooked it up and it was fine. Whoever did that was barking up the wrong tree on that car. Thankfully, because replacing that core sounds like a real bear.