I figured out the Mystery of the Wideband O2 Sensor Scaling.
I set the Infinity up to look at the wrong input.
It was reading one of the drive-by-wire pedal position inputs, so every time I stabbed the pedal the ECU thought the front bank was going super rich but when I was out of the throttle it thought the front bank was super lean. So! That pretty much explains things going pig-rich after start. Input scaling no longer required, it just reads properly now that I've specified the correct input. Imagine that.
Added ATF to the power steering system and manually moved the rack back-and-forth a few times. I'm sure it needs actual bleeding but oddly enough have never bled a power steering system before, so need to figure that out. Advice about this process / BMW PS racks appreciated!
Also, target of opportunity: cut, bent, drilled, and spot-welded in a bracket for the lower-left radiator support.
Now I just need to set up VANOS, which I may actually do after cam break-in because the car is trying to run pretty nicely right now and there shouldn't be any problem with having the cams in idle / start position at 2,200 RPM, right?