Thanks, Matt, as always.
This may have been my error. I have to put an order: -1 on the address selector field’s div in each of the shipping and billing blocks to get it to float to the top, and I may have missed that – without that CSS, it looks like the address selection function is not available where I expect to see it.
Anyway, Fluid Checkout also seemed to break my Address Autocomplete plugin (conveniently enough, they do sell addons for these two features).
I switched to a different single page checkout plugin, and both features worked right away.
Your plugin does indeed seem highly compatible Woocommerce standard checkout and with other open and compatible plugins. Well done.