My first suggestion would be to learn how to use Groups and Components more effectively. Your entire model is one gigantic Group! That causes every line to stick to every other line, just as if you hadn’t created a Group at all.
In general, the way to select something without getting things behind it or hidden is to single click the cursor on it. If there is a collection of attached geometry, double- or triple-click on it (double gets a face and all its edges, triple gets everything connected). When you drag-box select, you always get everything in context that satisfies the style of drag-box (left-to-right vs right-to-left) regardless of whether it is behind something else or hidden.
In your specific model, a quick fix that worked for me was to make the front Face a Group, open that Group for edit and push-pull the front Face back to the wall thickness. There are some missing lines in the windows in other walls that you need to add before the same trick will work on the other walls.
Hope that helps!
Steve