Are you dealing with raw geometry (edges and faces) or with groups and components? Any elements you have in a group will be isolated. If you try to erase an element that is part of a group, you’ll simply erase the entire group.
You’ll probably get conflicting advice on this point, but personally, I don’t start creating groups and components until I need to. As long as I’m doing basic geometric forming operations intended to produce a single, integral object, groups just get in the way. Everything you see in the pictures above is raw geometry; there are no groups yet. Now, once the final configuration of the object is completed and there are no additional forming operations remaining, only assembly, that’s a logical time to isolate and protect it by encasing it in a goup or component.
-Gully