• 1. Combat is measured base to base, just like everything else. So units on one level can't attack units on another.
• 1. Units can only sit places they physically fit. If, once you place them, it's apparent they are in danger of falling you can use the wobbly model rule to put them somewhere safe, but count them as still being in place. Can't physically put the unit there? Can't you WM to pretend you can. So no, you can't stop halfway up a wall.

In relation to your Tau problem - yeap, your DP can't charge if he physically can't fit up there. Think of it as the unit fortifying the 2nd floor to prevent you getting close. Your options are to either shoot enough FW to make room for you first, or charge something else.