Re: Other drives, trailer/ trike/ extra wheel....
I think a power trailer is a good option for a longer, straight run.
Probably not the best for turny, stop and go traffic situations...
Controls would be a bit more difficult to fab effectively.
IMO, flats are not an issue if one follows the Goat Herder advisory of heavy duty tube and a generous application of Slime.
I spoke with a local rider with a mitsubishi 50cc trailer mounted on a swivel, pushing his MTB. He said he's got 5 years on it and cost was about $100/yr for replacement chain and plastic stuff that got wore out or broken during use. He's a daily rider, with any number of longer trips about the county. ...and he's got a 10" kart wheel.
My goal is cheap, reliable, personal transport, below the tax collector radar. Fun counts for a lot!