Have you tried just taking the wheels off to fit a bike in the Mini? If you can fold the seats down, I would expect it to fit. Alternatively, I think most quality racks have locking ability, though I'm not sure how secure they are.

Everything I've heard about S&S couplers seems to indicate that they're designed to be used for the occasional airplane ride, not for breaking down after work and throwing in the car. They're not super fast (I've read reports of a full breakdown taking upwards of an hour), and the components are not designed for frequent decoupling/coupling.

If rapid disassembly is a priority, I would take a look at a quality folder like a Bike Friday. If, on the other hand, you can live without the collapsibility, I would look at belt drive bikes. Handlebars can always be changed, and disc brakes can often be added later, but I think belt drives are trickier to retrofit.

This website seems to have a pretty comprehensive listing of belt-driven bikes. Now you've got me drooling over some of them