I finished up my breadboard testing for adding a manual mute switch and status LEDs to this circuit. I wound up going back to the 2N7000 with a PNP as used on amb's epsilon12 circuit. I could not get the LEDs to work properly on everything else that I tried, as below:
Original circuit, with mute switch between relay coil and 2N7000 drain. Muted fine, but not enough voltage to drive the LED circuit.
Moved the 2N7000 to above the relay, in between +12V and relay coil. This would not energize the relay.
Here is the schematic of what I am planning on using.
And here is the resulting board layout, extensively cribbed from Dr. Gilmore's layout (though I did try some others first)
Still working on label clean-up, etc. Also, the BC 5xx transistors I used could probably be replaced with 2N3904 / 2N3906; just flip them around. I noted the transistor pin-outs on the layout, though they become slightly misaligned using Seeed Studio's CAM processor job.
I also put in both the Phoenix terminal blocks and Molex KK connectors. The mute switch and LEDs only have the Molex connectors currently, with a shared ground on the LED connector. I think I will switch to a pair of 2-pin or a single 4-pin connector and might add the Phoenix terminal blocks for those if anyone is interested. Board size would probably go up a bit with those as well.