1. Can I use the USB cable to connect the WebStation(USB Mini B connector) for power charging?

We indeed suggest you to charge your battery by the AC adapter included in your WebStation package. USB cable is currently not allowed to charge WebStation.

2. Why cannot I open the WebStation while charging battery?

For the safety purpose, WebStation required the lowest power-on limitation. If you charge the battery after using out of battery and ignore the recharging notice till the WebStation power-off, please recharge the battery to achieve the safety battery standard and you will turn on your WebSation after that.