I think I might've found a sedan company that has car seats. The one I've used in the past seems to have declined in recent years and maybe gone out of business, so I guess I'll be trying a new one anyway.
It's probably a decent idea to buy one to keep in Chicago anyway for future trips, but...feh. I could probably find a friend of a friend who had one to part with, but with the timing I'd need my parents to go pick it up probably, and they would just get annoyed at that. My parents are good people, and will take good care of V next week, but without armchair diagnosing them I will say I am concerned about their increasing-over-the-years unwillingness to leave their comfort zone/suburban enclave.
BTW I did run across a really awesome looking product for older kids (4+).. http://www.bubblebum.co/us/