Yes. It’s important to maintain the same polarity however. That’s easy if you just use a standard splitter like those offered by suppliers of CCTV equipment.
Not a good idea. Actually, you shouldn’t use a charger at all to power an IoTaWatt. Chargers can’t be relied upon to provide the clean power needed. Using individual 1A USB power supplies is recommended. There are plenty of inexpensive units sold for Raspberry Pi that will work fine.
No, recent releases of the IoTaWatt firmware do not need the voltage reference to operate. It’s not advisable, however, to connect the VT to a UPS.