I decided to create another revision of the PCB to correct the various mistakes in the first round. 

The original board dimension was about 94mm x 65mm. I did the layout as compactly as reasonably possible since I planned to house the circuit in a standard US electrical box. That compactness led directly to some of the problems I later encountered. I now know that the box I will use is 110mm x 190mm (minus some corner cut-outs). Since the inexpensive PCB fab houses typically have special pricing for prototype runs of 100mm x 100mm or smaller, I opted to spread things out and use the complete 100mm x 100mm for the layout.

The new board layout is in the project: https://upverter.com/design/wjcarpenter/oelscv2/. The schema also has minor updates.

Besides the more relaxed layout, the revised design has:

• pins for the I2C bus so I don't have to tack onto pins of the microprocessor board
• pins for the single previously-unused microprocessor pin, possibly to be used for an audio alert buzzer
• corrected wiring for the MOSFETs
• external pull-up resistors for the pins that need them (easier than rearranging the pin assignments)
• current-limiting resistors for the PWM signals to the MOSFETs
• replaced the 2-pin screw terminals with euroblock connectors
• top-layer copper pour for +3v3 (in addition to existing bottom-layer GND pour)
• wider traces for the +12v and return paths