"The electrician keeps telling me he can't do this, that the motion sensor needs to attached to the light fixture itself"

No, the motion sensor can be separate. This TREEWELL Motion Sensor is an example of such a product. I would be sure to set the sensitivity low so lights aren't unnecessarily turned on when activity is far away.

Be sure to follow Dark Sky Principles for your outdoor lighting.

Minimize outdoor lighting. Enjoy the night sky. See the stars.

So how many separate "circuits" do you want? And you want each circuit to be controlled by a motion sensor - or by a day/night sensor - or by a manual switch? Or do you want one switch and day/night sensor to control all the circuits?

i'd love to see the wiring diagram for all of this!

The TREEWELL Motion Sensor should be wired to a wall switch. By following these instructions, you can manually turn on the lights, bypassing the sensor (i.e., auto mode):

"To activate, turn the wall switch OFF and then ON within 2 seconds. The light will remain on for 8 hours, then automatically turn off at dawn. To return to Auto Mode, turn the wall switch OFF for more than 2 seconds and then ON."

That "simple... solution" is commonly found on many if not most motion sensor lights...