Pouchaiquou87
P2P tickets are always for a route, on whatever trains using it. So yes, along that Montreux-Geneve route, any intermediate stop and continueing with another train is absolutely fine.
If checking the online timetable, you will notice that in the overview, the indicated (single journey) ticket price is the same (from CHF 16.00) for all connections. And if drilling down to the respective suggested ticket, it would always be exactly the same. If one of the connections would take a different route (which is not the case here), that would be reflected in a different ticket price and therefore different ticket.
Usually there are many direct trains back to Geneva also later in the evening. That transfer in Coppet to a replacement bus is just related to temporary track closures (engineering work) next weekend. Existing tickets are always fine on official replacement (bus) services.