As you appear to understand the ND monthly pass is good for a calendar month; during that month regardless of the time of year, you can use it for any zone on weekends. e.g. if you had a 1-2 zone pass, you could go to Disneyland, or Versailles or whatever on the weekend even though they are outside your zone.
I don't know the dezonage rules during July and August, but I assume those will be on the train site. The dezonage on weekends applies to monthly but not weekly passes. I would assume their weekly passes will not have this feature in July and August, but they should check with the authoritative site for that. A lot of tourists use weekly ND passes. This is not a product designed for or marketed to tourists and so one way they make it less attractive is to not allow the dezonage features for the weekly pass. Locals who use the ND, charge it monthly and can avail themselves of these features as of course locals with the regular Navigo passes do.
For your left over days you just get regular tickets. If you are in zone 3 and want to visit Paris on those days then you will need a specific point to point ticket from your RER station to Paris. You could buy a carnet of 10 of these tickets and get a reduced rate. Each ticket would take you from Colombes to anywhere in Paris. Once in Paris you would need additional tickets to move around on the bus or metro. I think I would plan those days so you walk when you get there i.e. head for the Louvre and spend the morning there and then walk to other sites in the area and then use another of your Colombes-Paris tickets to go back to your base. These tickets are good forever, so if you buy a carnet in July, you could use leftover tickets in September or next year. Save zipping around from one end of Paris to the other for days you have the ND pass and save trips to Versailles, or Disney or Auvers sur Oise or wherever for the weekend dezonage on the pass.
If you come into CDG then you would get a single ticket from CDG to your place in Colombes. A CDG to Paris ticket is 9.75; I assume a CDG to Colombes ticket would be a bit more. It will allow whatever transfers needed to get to your place. If on September 1 you need to get to the airport or a train station in Paris then you would need the specific point to point ticket to do that; if it is a train in Paris then one of your Colombes-Paris tickets would do it -- if you need to get to one of the airports, then you would get a specific point to point ticket from Colombes to (airport)
