If you need the stretched portion to be a different colour, make this wall using a complex profile. Do this by selecting the existing wall, right-click, then select 'Capture profile of Selection'. Add an extra node in the polygon where you want the change of material to occur, then edit the material of the part of the wall below.
Question 2 - Personally I would model the walls separately from the foundations. This will give you better control over what is displayed on plan.
Put all foundation and substructure elements on a new storey below ground level. Set all these elements to display on home storey only then they won't interfere with the ground floor plan. These elements should be on different layers too for maximum control.
In question 3, are you asking what level of detail you need to model? Usually I model all features that would appear on a 1:50 scale drawing. I would then enhance larger scale details extracted from them with 2d lines, fills and library parts.
Hope that helps!
