A380 window seats
 

There is only one A380 operator - but two other airlines plan to start soon and some cabin specifications have been released.

Is a lower deck window seat (non-suite) comfortable?

The sidewalls slope outwards above, unlike on the other widebodies.

Is it feasible to lean against fuselage sidewall, or does it cause the passenger to slip into the seat behind?

Is it possible to make the window seatback wider so as to close the gap before wall? Singapore does not do so - but will Emirates or Qantas?

Also, does the outward sloping sidewall offer better views from windows downwards?

When you considerthe overall size of the aircraft, I doubt that the degree of slope will be hugely noticeable.