Road congestion and travel

[CheerfullyInsane]

It seems like these both apply to whole divisions though. 

No, it's just awkwardly worded in the manual.
What they actually mean is that Immediate AI Orders apply to all units of the same formation in the selected,and adjacent hexes.

So as an example, let's say you have a division in a town.
Three regiments of three battalions, each regiment in its own hex, all three hexes touching each other.
By selecting one hex, holding down Alt and right-clicking, the AI will move the units in T-mode in sequence, starting with the selected hex, then moving on to the next hex, and so on. So you end up with a column of troops all following each other.

The main issue (for me at least) is that you have no control over the sequence itself, i.e. you can't affect which units move first. So what I usually do is use one or two turns setting up the column myself with the marching order I want, and then start using Immediate AI Orders. Which also allows me to send out recon units in advance.

Final thing, if you're worried about the AI path-finding (and you should be ), you don't have to use immediate AI Orders on a distant hex. You can simply Alt+right-click each hex in sequence, and you get this conga-line of troops following the lead unit. Slightly more time-consuming, but far greater control.