Greg, the two-screw romex connector is almost certainly also listed for SO, though I also would prefer a connector with better strain relief.
There are two different strain relief test standards, but the code does nt address the issue. UL standards do address this point, and I'm pretty sure they would also require a better grip; but as far as the NEC is concerned, it gats a 'pass.'
I do not think it's three-phase; I'm pretty sure that panel is available in single phase only. (After all, there's only one row of breakers).
Tom, I think we can find the GFCI requirement when we apply Article 525 (Carnivals and Fairs) to this gizmo, though the GFCI can be part of the plugs on the cords, rather than built into this assembly.
