If I use the auxiliary power (input 2) to power a Dev-14150 and then plug in a micro-usb from the relay plate into a Raspberry Pi 2 micro usb power input, would it be able to power the raspbery pi?
That should be fine (though their documents don’t have a schematic!) https://pi-plates.com/relayplate-users-guide/ - wire up the input2 power (be sure to move the jumper), and either use a voltmeter to confirm power is getting to the micro usb, or just plug in the Pi and see if it powers up