As I know, in most US states E-assist is limited to 750w output power vs. 250w in the EU.
Max speed is 20mph (32km/h) vs. 25km/h for an EU pedelec.
BUT you must not have four wheels, if motorized.
Maybe, the push trailer you mentioned is the best solution for this dilemma.

Another idea, I'm not sure if this is acceptable:
Here in Germany, a very narrow track counts as single wheel by law.
As I remember, the max track width which applies to this regulation is 450mm/17".
Does a similar regulation exist in the US???