Problem solved! First, for some reason along the way I started to use '/etc/acpi/wakeup' -- which of course is incorrect. Sure enough, '/proc/acpi/wakeup' was still intact.
Here are the steps I took to allow my USB keyboard and mouse to wake the computer from sleep and to make sure that these settings persisted after a reboot:
In terminal, type:
You will see a list of devices. Note the USB ones (named as USB, USB0, USB1, etc.). I only have two, so instead of going through the trouble of figuring out which one was the keyboard/mouse wireless receiver, I decided to enable both. In my case, this was USB and USB2.
In terminal, type (and adjust 'USB...' to match the names of your devices):
Now you should be able to resume from suspend via a USB device. In order to force these settings to stay upon reboot, you need to add all of the 'echo USB...' lines that you used above to /etc/rc.local so it executes upon boot.
echo USB > /proc/acpi/wakeup
echo USB2 > /proc/acpi/wakeup
That should take care of it!