In which state are the configured UART pins if the device is held in RESET for nRF52832?
We want to use the UART to communicate with the host without the nRF52832 in certain cases.
Also, if the device is unpowered, can the UART be used with the host (not using nRF52832 of course), or will this likely damage the device as it will exceed the Abs.Max rating for I/O pins, V_I/O < VDD+0.3V?
When you hold the chip in reset mode the GPIO pins will be configured as PIN_CNF[pin number] = 0x02, which means:
PIN_CNF[pin number] = 0x02
For debugging purposes, you only need to connect the debug-device on the host TX line/nRF RX line, which you can safely do when the nRF is in RESET mode(as long as you don't exceed the Abs.Max rating values VDD+0.3V). You should not apply any voltage on the pins when the nRF is unpowered, as this will exceed the Abs.Max rating values since VDD is 0.
Can the default behavior be configured to be different than the above?
No - the default is hardware defined.