There are two kinds of forest...
a) high density where no unit may move across the area
b) low density where there isn't any problem for units movement.
Speed...
The different speeds are in a French Study : Des Marches dans les Armées de Napoléon*, Lieutenant C.P. Escalle (1912)
This study talks about the French Army organization. Escalle has studied a lot of documents on the napoleonic era: Alombert et Colin, Berthier, Davout, Fabry, Foucart,... So, I think it's not a bad idea to use these parameters for HW. For other countries, because I don't find any precise informations, I adjusted the values from the French ones.
What does this mean:
"Is the units in a Corps? In this case, the speed is one of the slower unit."
OK

With the orders deploy and diversion, all units must move at the same speed.
So, the AI uses the lower speed of units in order to avoid units don't follow the movement.
Clear?
JMM
* March in the Napoleon Army.