Bohemond,
Sounds like some interesting suggestions to improve the 1815 scenario and NCP overall. You've been doing great work helping to improve NCP and correct lots of little booboos which will make NCP much better!
I like the concept of different garrisons for different structures, reminds me of an old saying of my generation - Different Strokes for Different Folks!

Basically line infantry should be in larger more important areas while militia goes to lesser areas. I also like the idea of putting them in on the first turn if not in the scenario setup, but wouldn't that require adding more replacements to the scenario setup to ensure there's plenty for all?
I still think that for the 1815 scenario the map needs to be cropped to match the playing area so we don't have garrisons showing up in say Nice which is way out of the playing area. That's what really annoyed me was seeing blue units on the global mini map that I couldn't access through moving around the game map within it's preset boundaries. Deactivating all those superfluous areas would go a long way to reducing the wayward garrison problem.
Which brings me to another point which is to make sure the playable map at the highest zoom-in level allows players to see all of their units. In 1815 it's tough to see Amsterdam and in the 1805 Sun to Austerlitz scenario Brest Litovsk is tough to see and access those Russian units unless one zooms out to the highest level. Not something I expect to see fixed in NCP but hopefully NCP2 will be better at giving us the whole playing area within the boundaries of where the map goes by just moving it around.