I'm happy to announce I have found some answers to my own question:

1) The wheels are really not hard to get off, or to put back on. Ian Sims, the developer of the Greenspeed GT3, reassured me. Takes maybe 5 minutes to put 'em back on, plus the 3-5 minutes to unfold the trike and to put the steerer bars back in position.

2) If I fold the GT3, take off the wheels, and loosen and compact the steerer bars, the GT3 will still not fit in my Honda Civic Hybrid's trunk, at least not without taking out the boom. But it will fill in the backseat of the car. Good thing I have some good upholstery cleaner so my wife's not wise to me, but next time, I'll line the seat with a tarp. Total time is about 10 minutes to take down the bike, and 10 minutes to reassemble it.

3) Possibly quicker solution: I bought one of those rear carriers that goes on the trunk. I fold the GT3, and put its cross member across the arms of the carrier. That is a bit faster, if a bit less secure against people rear-ending my car and killing my trike.

In a few weeks, I'm headed 90 miles away to meet a friend for a ride. It will be fun to arrive in the Civic...