That’s correct about C17 being in the same circuit as Q3. So if there are no shorts across the pins of C17 then Q3 is not the main problem. It is still not behaving correctly because of your voltage measurements, but that’s not the thing causing no signal to pass through the board because C17 is removed.
You are right, it could be the relay. Go through steps 5-7 on the troubleshooting guide (assuming it passes all the steps up to that point). If that doesn’t help you may consider following the steps in appendix A to see if the LEDs in the optocouplers are working correctly.
-Brach