1. Walls are always measured in their middle but you can use the ability to compute values in numeric text fields to help you for the maths. Just enter a math expression and it will be replaced by the result when the focus leaves the text field (see Tips section in the help for the allowed operators and functions).
If you need to draw walls abutting along vertical or horizontal walls precisely, you could also use alignment lines which also appear along wall sides when you zoom in.

2. If questions about measuring walls in their middle have been asked many times here, I think you're the first person to ask here if there's a standard practice about aligning walls.
If you want the exact volume of plaster or concrete needed to build a house, joining walls along their borders or at their middle could make a difference, but I'm not sure that tolerances in buildings could make that difference important and wonder if it's really the goal of Sweet Home 3D.
Personally, I generally draw an abutting wall from the middle of the wall it's joined to, using alignement lines shown in wall middle. If a measure is important, I think it's better to add a dimension line than relying on how walls in the plan were drawn.

What do you mean by "odd corner"?