EDIT: I have misread your question, your assumption is incorrect. See below
On the first question, your assumption is not correct: weekends are "unzone" periods in which your 2-zone Navigo Pass can be used for all zones within the Navigo zones. Other "untouched times" are holidays, school holidays for Ile-de-France and in summer (mid-July to mid-August). Source. NOTE: This is Only valid for the monthly ticket, these does not apply to the weekly ticket,
EDIT: does not allow you to use the pass for destinations outside the Navigo tariff zones. The suspension is only relevant to the 2-Zone Pass that you can purchase (by opting out of the All-Zone Pass).
The second question does not prevent you from doing so and you save money by using this scheme. Just make sure you have enough time to get the new ticket, as some destinations are overcrowded and you may have to wait up to 10 minutes before you can buy a ticket. This is called a "complément de parcours" source