Planning a few day trips in Paris from Le Marais area to Versailles, Fountainbleau and Paris Disney and thought I would buy a pair of tickets for each trip beforehand so my round trip would be taken care of. Is it allowable on my return to modify my last few METRO/RER segments to go to another area in Paris still within the two hour time frame per trip?
For example, can I just change the last stop of my return trip near the Eiffel Tower or Sacre Coeur instead of my Marais original starting point?
Will a pass of some type, a Navigo fare for example, not work better for you than individual tickets for multiple trip segments?
For the purpose of tickets origine-destination, all of the métro and its corresponding stations is considered to be Paris. When using passes, all of the métro and its stations are considered to be part of zone 1 (even through physically that is not the case). Tickets origine-destination are sold to/from Paris and a specific RER station (or from RER station to RER station).
The two-hour validity limitation applies to tickets t+ and specifically tickets t+ used for métro travel. I am not familiar with any timed restriction for tickets origine-destination.
When using the t+ for bus travel, there is a 90-minute limitation between the first validation and any connecting validation. For bus travel using the t+, there is no maximum time of ticket validity. The restriction only pertains to making bus connections.
You can return to any where in the Paris Metro system. Going, you use the ticket to enter the Metro system, then when you transfer to the RER, then to exit the RER at your destination. Returning, use the return ticket to enter the RER, then when you exit the system inside Paris proper, You may need it to reenter the Metro system depending on the station, but you do not need it to exit the Metro system. There is no way of knowing where you exit the Metro.
when Paris is on one end of this type ticket you can go anywhere in Paris; what you can't do is return from another destination. I learned this the hard way thinking the ticket would work in the same zone -- it doesn't -- it only works from the particular station and tickets cost different amounts to various stations in one zone. Zone doesn't matter with these tickets. But Paris/Versailles Rive Gauche gets you to and from any station metro or RER within Paris.
And if your dates allow a Navigo Decouverte would be the way to go here.