No. Let's say you ran the test noise and all but one channel came out at 75 dB, and one was 70 dB. You have two choices: a) raise the gain 5 dB on one channel, or b) lower the gain 5 dB on all the others. Either way they all match.

However, the volume control display does not represent the same playback loudness in both cases. In case a) a given volume setting will be 5 dB louder than it will be in case b). No matter, it's only numbers. And when you crank the volume to max, it will stop at "-5" on case a) and at "0" in base b). But the SPL will be the same in both cases.

Not if the reason for the cut is to bring speakers into balance with at least one speaker at trim = 0 or greater. But if you set all the trims to -10, then you will reduce max volume 10 dB because the system will only go up to 0 and stop, rather than go up to +10.

So if anyone is applying attenuation to every speaker to balance the system, that will reduce max volume. Best to increase all of them by the same amount, until at least one speaker trim is at 0.