It's just too coincidental that the problem followed you between two Spark EVs. If the only third-party accessory you ever have installed in the car is a cell phone charger, I'd pull it out of there.
As for reports on the Volt forum, I've been a member there for 10 years and read it almost daily. Every time there is a report like this about brake failure, it's been an issue with a third party accessory causing CAN bus interference. And, just like this case, the brakes didn't fail completely they're just in manual mode (pedal drops till you're manually activating the friction brakes). I understand that's scary and requires more effort to stop, but it's not complete brake failure.
If five people at your dealer have had brake issues, you should all open NHTSA cases and all call GM to complain. That's would be an unheard of number of braking issues with a single car model in a small area. Did you buy both cars from the same dealer? Did they install any dealer add-on accessories in both cars (car alarm, LoJack, backup camera, or anything else weird that the car didn't come from the factory with)? Not to harp on it, but interference from aftermarket electronics is always the cause of these braking issues.
I hope the dealer is able to determine what the issue is and get you back on the road and feeling confident in your Spark EV.