Marines/sailors in each Division and a pontoon in the same stack also work together to get the Divisions across the river most quickly. You can test this by plotting a move with different stack configurations and seeing the number of days required. The overall difference depends on the moving unit's cohesion, leader activation and the terrain of the region across the river, so I don't have a hard figure like X number of days saved. MTSG units don't get a river crossing penalty. The combat penalty is different from the movement penalty. A river crossing is treated like an amphibious assault. If your units don't do well in the ensuing battle, some or all of them may end up annihilated. I posted in a thread that one might sneak a Division and a Corps commander across a river in small groups using P/P and then assemble them into a "Corps" the next turn. They could attack without the river crossing combat penalty. The rest of the army would MTSG and also suffer no combat penalty. I believe that the game engine may have been tweaked to prevent this.
You should give militia every advantage you can. Put them in battles where you have a good probability of winning and they should gain some experience. In one battle of our Revolutionary War, the militia were instructed to at least fire three rounds before they ran off.

I'm the 51st shade of gray. Eat, pray, Charge!