If I drag a window or door, I get little hints as to the relative position. Is it possible for plugin to extend those little blue hints to wall movements?

Nope, you have to calculate this yourself. Unfortunately this is an example where using the imperial system doesn't make it any easier. With the decimal system it's pretty straightforward with any dimension, using the imperial system is only easy with 'easy' dimensions.
The number fields are little 'calculators': you can enter a formula. For example (5.5/2)+1 is resolved and the result is set as the number. I don't know how well that works with the imperial system.