Sounds like progress!

 

If the output voltage drops under load, how do I tell if it is:

1) bad board layout , not enough ground planes etc
2) insufficient coil
3) insufficient diode
4) something else ?

I have set the output to 3.3V. Input 27V. Connected a 2R2 resistor as a load, output drops to 2.9V. The switch output looks good, a normal square wave. If I decrease the Vin, the duty cycle increases as you would expect.

The heat on the output load was 75C. The diodes (3 * SB1100 in parallel) all at 40C each. The LM2679 with a medium size cooler at 30C. The coil itself stone cold. Ambient 19-20C.

 

I would use a single SB5100. Diodes don't play well in parallel.
As the load increases and the voltage falls the duty cycle should increase just like when you drop the input voltage. Might be some noise on the FB line. They warn about that in the data sheet.