My advice was not to ignore the manual. I was simply stating (maybe using different words) exactly what we have written here:
http://mainsequencesoftware.com/Content/SGPHelp/UnderstandingAutoFocus.html
Can't really say. It is a function of exposure time, download time and data points. Fairly easy to calculate if you have all that data. What I can say is that a typical exposure time for AF is 5-10 seconds. We don't recommend running AF with a narrowband filter... that's just trouble waiting to happen.
Minimum HFR is a relative measure based on your equipment and the environment. It's not impossible (or important) that 4.8 is the minimum, only that you have a "V" or a "U" shaped graph.
Different focus motors do different things. Some motors have a very fine step size and will need higher step sizes. Some motors have very coarse step sizes and need lower step sizes. In terms of physical distance moved, 125 and 2 steps could be identical.