“On a half-normalled bay, what is the electrical effect of splitting the audio signal? In other words, how does the level and impedance change when the output signal is patched for monitoring while the normalled signal continues on its path.”
What you are talking about is really no different than a common “Y” cable. In this case one output has to drive two inputs, which is like connecting two speakers in parallel to one output of a power amp. Two 8 ohm speakers in parallel present a 4 ohm load. Two 500k ohm input impedances in parallel represent a 250k ohm load. In these line level electronic devices, as long as the net input impedance is at least twice what the output impedance of the sending device is you aren’t going to load anything down or cause problems. You can look all these values up if you want, but the bottom line – and the short version of this answer – is, with most of the equipment you use this isn’t going to present a problem. The input impedances are so high compared to the output impedances you could probably put 10 or 20 things in parallel across one output without significant problems.