I've had the same thought.
And the same questions.
The drawback of putting a small wheel in the back is how to raise rear end back to normal pedal clearance height.
Hooking the pavement is not just annoying but can be a safety concern.
That's where a 29 inch fork comes in for the front I presume, to compensate for that.
If you don't plan to pedal much & they're just there for legal compliance then I suppose you could just go with short baby cranks.