Not sure about your first problem. Sounds tricky.
As for selection, you have 3 selection methods available. Partial Elements, Entire Elements, Direction Dependent (left to right, image below). If you have Direction Dependent set, when you drag your window from left to right things must be entirely within the selection window to be selected. If you drag from right to left elements need only cross the selection window to be selected.
Another selection method which is quite handy is to combine a Marquee with a specific element. So draw your marquee, change to the tool of the element you wish to select, and CMD-A (or CTRL-A for win) to select all of those elements within the marquee.
Note also that if you are trying to select a particular element, make that element's tool active first, then selection will default to it. For example, if you're trying to select a slab which has an edge common with a wall, select the Slab tool first and the wall will be ignored, the slab will have priority.