1.Some spearmen units are mean't to be better than others.

2.The Druz have an AP bonus due to their axes. Which means they have more attacking power versus anything with armour. Also Boyars have javelins for varity more than anything. Instead of mailed knights the Poles Hungarians and Russians first noble unit is ranged. But only the Hungarian one is a HA (as they get no more HA after that). The Russians get 2 more, and the Poles are more western leaning.

3.The Druz AP bonus is key here too. And they are cheaper to maintain.

4.Your putting way way way too much stock in the name alone. I larned with TW to never put much stock in names. Light raider HA are common for eastern factions. The name doesn't matter overmuch. For paly ballance the Russians need them. They probably chose Kazakh cause it was eastern sounding and easy to say. Cuman is almost dirty sounding (sounds strange ut it's a concern for rating purposes) and the Russian form of Cuman is too hard to pronounce.

5.And you've just made your biggest mistake right here. This isn't Kiev Rus your playing. Or Novgorod for that matter. It's the Russians, as a cultural/ethnic group. Not 1 Russian princely domain but all that existed between 1080 and 1530. Look at it as if your playing Kiev, Novgord, Muskovy, Smolensk, etc at once. CA has always done this made factions out of a cultural groups (this was much more common in RTW with the barbarian factions) when you have a cloud of small states with more or less the same military and culture. And there is no point (game play wise) in splitting them up.

6.Again not Kievan Rus your playing. Also the Provinces are almost exactly like MTW. And there is a set limit to how many you can have. So areas that had more fighting over them have more provinces. Like the Levant, Iberia, France, and Northern Italy. The steppes in M2TW are a lot better than in RTW. More provinces and they are easier to get to.

7.The tech tree is fairly standarized it would be very unballanced for the Russians to not have castles. Hell the Milanese and Venecians need to use castles when historically they never would have.