Re: When are we done with boosters?
Besides passing the 5 step test, it is somewhat important that a kid have started puberty before riding in an adult seat belt without a booster. I'm not a biology expert, but there's some way in which a person's hipbones change during puberty that means they will be better-protected by an adult seatbelt. With "kid" hipbones, they're not quite the right shape to hold a seatbelt in place on their own, and a booster acts as artificial hipbones to help out with that.
All that said - maybe a bubblebum would work?
And to your last question, I'd be okay with the 5 year old riding in a parkway as long as he stays put and doesn't mess with the seatbelt or move out of position. That's a tall order for most 5 year olds, but some are ready.