Why squirrel behaviour varies greatly throughout India?
Given that you're dealing with fairly smart mammals with a short lifespan and limited distance migration, it seems likely that you're dealing with strong local selection effects that are going to be reflected in all three layers:


Learned behaviors from observing other squirrels and their own interactions with humans.
Epigenetic effects tied to things like maternal stress
Differences in the frequency of genetic tendencies towards caution / aggression, etc.


Now, what would the source of those differences be? I don't know if anybody's studied these particular cases, but there's some pretty strong candidate hypotheses for a starting point:


In West Bengal, you're dealing with high-density non-touristy areas (especially Howrah, right on the edge of Kolkata). The squirrels there are likely in constant conflict with humans who do not want to be sharing their food, which will select strongly for aversion to humans.  Humans might well be poisoning them as a means of control too, which could explain the picky eating.
In tourist spots anywhere, the squirrels are probably getting fed well by humans either intentionally or accidentally (e.g., careless children), whcih will select strongly for attraction/aggression toward humans.  I'll bet you'd see this in Victoria Memorial too, for all that it's right next door to Howrah.
In Uttarakhand, which is a much lower density state than West Bengal (189/km^2 vs. 1029/km^2), you're probably seeing much more of the default state of interaction, without a strong selection in either direction.