The Niles speaker-level volume control offers impedance magnification settings of 1x, 2x, 4x, and 8x. According to Figure 7 in the manual , you should set this based on the number of pairs of speakers driven by that volume control. For a single pair of speakers (either 4-ohm or 8-ohm), you should set it to 1x. The 8x setting is used only when driving 9 to 16 pairs of 8 ohm speakers, 5 to 8 pairs of 4 ohm speakers, or some combination thereof. Basically, it's multiplying the speaker impedance by the amount listed (with 1x being no change).

The 7075 is a pretty robust amp, so it really won't care if it's getting a 4 ohm load put on it. The manual for the Niles SS-4 selector switch (which was the first one I found that had a protection button) recommends using the protection mode when driving three or more pairs of 8 ohm speakers or any combination of 8 ohm and 4 ohm speakers (page 3 in the PDF file). If you had the kitchen and the mains going at the same time, you wouldn't need to protection mode. If you had both of them and the patio, it'd probably be best to have the protection mode engaged. As for what the protection mode does to the sound quality, I don't know. I guess you could engage it and turn on all three zones then listen to the main system a bit to see.