Welcome to PA2V2 world.
1. From empty, how long to fully charge(I'm using usb cable-usb plug-power outlet)and how does one know when it's fully charged?
A. You don't. And that is what we hope in the future release. I use external battery recharger and it has logic to shutdown when fully recharged so I don't know how long it should take from the usb cable or AC adapter the amp comes with.
2. Mine came with ordered rechargable batteries but within six hours light went out and sound failed - so battery flat? Normal?
A. Mine lasted for quite some time but it could just be a luck on how much battery was remaining when it was placed in your amp.
However, I did full recharge even before it drained. I used a battery charger instead of the one that came with the amp. I am not using this anymore(in my bag dusting) but it did go over 150 hours. My batteries are 2500mA each. After over around 150 hours, I just recharge because I don't know when it will die.
3. Now recharging but what if I forget/get distracted? What damage can over-charging do? This worries me as I'm used to recharging my Zo being a breeze(light goes out, won't overcharge)and this is for my dad, who, with the best will in the world, may forget and leave it too long. Why does it overcharge?
A. Because there is no logic integrated to the amp design. Maybe the developer did not think it was worth to add it based on how much more expensive it had to get.
But if you are like me, I don't mind opening up the case one a month to recharge the battery. Otherwise, just recharge for 12 hours while powered on and you should be good. Overcharging shortens the battery life. It shouldn't damage your amp unless the batteries get boiling hot and the heat damages the amp(?).
4. Is re-charging via usb cable into mains outlet analogous to using AC adaptor? ie, if I leave unit turned on whilst charging, will it NOT overcharge?
They do the same, usb port from your PC would have lower amperage so it can take more time to fully recharge, since we will not know if it is fully recharged, I would rather recommend using the AC adapter(while powered on!) and run it for 12 hours.