Ok, let me just stick my foot in my mouth a little further, and maybe tackle #3 and #2 in your list, Marvin:
If it was possible to supply GPS coordinates for some key points in the synth, would this help with the hints and hand tweaking of parameters?
Yeah, more or less what I'm getting at is being able to take a whole collection of ortho or near-ortho aerial images, maybe a set of obliques to compliment them, and a set of ground control points, and doing orthorectification, georeferencing, and photogrammetry off the resulting mish-mash of data. I realize this is outside the scope of what Photosynth is about, but a lot of the work has already been done.
In my dream world, I'd love to be able to go out with a bucket of tennis balls, lay them out on a grid, grab the GPS coords of each ball, photograph the site from the air using heavily overlapped shots, and then toss the whole lot into a magic black box that spits out a 3D model. (I'd pick up the balls.)