the UART can be used by an other connected device, if the terminal console of the StromPi3 is in its silent mode (default at boot of the StromPi3 or through the use of the „quit“ command). Please make sure that all screen serial sessions are closed to free up the serial interface.
If you want to use the StromPi3 as an UPS which shutdowns the Raspberry Pi in a powerfail event, then you have to connect one GPIO Line of the RPi with the StromPi3 – this procedure is described in the PDFs (in english and german version) you can find at the following link at our github https://github.com/joy-it/strompi3/tree/master/SerialLess-Mode
There you can also find the modified versions of the Scripts to use them without the serial interface.
PS: Wir sprechen/schreiben auch in Deutsch 😉
Yes, all of the GPIOs of the Raspberry Pi are forwared through the header on the StromPi – no GPIOs are blocked and can be used in normal operation; only when you are configuring the StromPi 3 through the serial terminal console, then you would have to use the UART interface of the Raspberry pi. But you can deactivate the configuration terminal interface through an extra command, so you can also use hats, expansions or ICs which are using the same UART interface. Please refer to the english manual in the downloads section on page 9.