[lexicon]

If you only get assist above 3k then there's something wrong. I wonder why you'd be using that many revs in 1st or 2nd in the city anyway (if that's what you meant). I almost never go over 2k in normal town driving and use the torque of the IMA.
The thing is, assist is more to do with load than revs - it could be that you're actually too light on the throttle and not demanding enough power to get assist.
Try using sport mode and using a little more pedal and see what you get then - should be lots of assist.

 

[lexicon]

It will pull pretty well from low revs in almost any gear let alone 1st & 2nd. The IMA gives a slug of torque low down so you need to adjust your driving style to benefit from it - it's almost like driving a diesel.
The shift indicators do a reasonable job of guiding your gear selection, it will tell you to shift down if the engine is labouring.
Not sure about more assist=less fuel - the two work in tandem. You want to accelerate reasonably quickly up to speed, get the benefit of the assist and then back off the throttle.
I know what you mean about Sport mode - it's almost too sensitive, especially if you've been in Econ.