Another assumption, yes, but it's a pretty safe one - it's very likely that for a given ISO setting, the noise recorded with no light input will be the same from shot to shot (under a controlled temperature), and also very likely that for a given ISO setting, the number of photons required to fill a well will not vary significantly from shot to shot. So, combining data from sequential shots is reasonable.
Your numbers are wrong (or rather, they are correct for an 8-bit capture, but not for the 14-bit ADC that's commonly used now, where the upper bound is 214-1 instead of 28-1, i.e. 16383 vs. 255 for white).
As for how dark to get black or how bright to to get white in one scene, that's going to depend on the ISO selected for that particular shot. The calculation determines the maximum possible DR for a given ISO setting (and DR decreases with increasing ISO).
In your example, if you exposed at median value, a camera with a narrow DR would give you blown highlights and blocked up shadows, while a camera with a wide DR might not clip on either end, if the DR of the camera exceeds that of the scene.
Yes, that's pretty much what I mean by 'usable' DR. It's relative to the exposure, but wider means you can capture brighter highlights and deeper shadows in the same scene, and narrower DR means you have to choose which way to bias your exposure so you sacrifice either shadow detail or highlight detail.
An alternate way to use the step wedge, and how DPR does it IIRC, is to expose so that the middle of the wedge is exposed to 18% gray, then see how far up the wedge from there until clipping occurs and how far down the wedge from there until the difference between the steps is lost in the noise.