Would one ignitor affect the whole circuit?
This the wrong thing to ask. Keep in mind what the question tells you — namely that voltage doesn’t seem to be getting to either the bake or the broil ignitor. We’re not trying to test the ignitors themselves. We’re trying to test their power supply.
The first step in doing this would be to see whether the relay board is sending out voltage to the ignitors. How would you go about doing that?