1) Not sure I understand your question. Could you clarify it?

2) From https://drawabox.com/lesson/6 "(don't switch pens to do any sort of "clean-up" pass - use the same pen through all your lines, including construction/box subdivision/etc)." For L6 and L7 it's recommended that you use ball point pen. These pens give you a bit more flexibility as to line weight. By pressing lighter or harder you can get varying line opacity. You can then use lighter liners for boxes/subdivision lines and press harder for constructing line

3) Yes, you can use ruler for everything. In fact it's recommended that you use all types of tools like ruler, french curves etc. Sometimes, you might find yourself having to freehand stuff. In which case remember:

"Whenever drawing freehand, I still want you to apply the methodology I've outlined in the past - the ghosting method, drawing through ellipses, and so on. That said, in this case it is inevitable that with all of the necessary construction lines, and the significance placed on precision, it's important for you to be able to use tools that will allow you to focus more on the meat of the lesson, which is really about the manipulation and construction of complex compound forms."