Pretty sure this can be modded one way or another.
Based on what exactly? I don't see a way to "fix" this with mods other than adding new provinces.
No, much simpler - bonuses, levy multipliers. You can just set them as per region, because India is one region. And then those little green multipliers will apply to every single Indian province.
This !
I made this suggestion in a thread about China and even did a bit of planning on how to implement it. Since most of the population disparity can be explained by there simply being more "hydraulic cultures" in the east all that would be needed would be to add say four new buildings to Castles, Cities and Temples - Drainage Ditches, Dykes, Canal Network and Grand Canal. These would be limited to traditional floodplain areas, eg the Ganges, Indus, Tigris/Euphrates and Nile and would add a MASSIVE amount of ultra cheap and ultra crap levies, say 1-4,000 Light Infantry with a -30% morale malus, PLUS a massive tax income say +5 to +20 pcm each based on level.
Against this would be a series of events that called upon the player to pay to keep the canals clear, say 250-1000 gold per pop, say once every 5-10 years on average. If the maintenance wasn't kept up then the relevant improvement would "silt up" and disappear, while at the same time creating a massive general opinion penalty (didn't maintain the realm) AND a truly frightening peasant uprising, say 10,000 troops per level of improvement.
The improvements could also be limited to certain government types (eg Feudal, Chinese Imperial) and maybe also certain cultures so that Kirghiz nomads couldn't just waltz into Baghdad and start running the waterways, at least until they settled down.
I'm actually going to try testing this out and see if it works. The combination of truly massive crap morale armies plus periodic rebellions and societal collapse should hopefully be sufficient to see great nations arise that are literally too big to project beyond their borders (how do you move 90,000 -30% morale LI through the desert ?) while also requiring a massive reserve of cash to cover the expected ongoing waterway maintenance costs.