Ok we just recovered it using the 'hard reset' via super useful post: Particle debugger to recover electron? - #11 by ak71vie
In short -
*'Rolled back' to tinker@2.2.0, came online fine.
*Then local flashed with an 'old' firmware, also came online fine.
*Then separately (repeating above) cloud and local flashed to 'new' firmware (running fine on other ~18 devices), and hard faults again...
Curious what to look for, since in our minds faults here but not on other devices, all things apparently the same.