There is no one at Interrail that you can call but the Community is very good at giving advice regarding reservations.

Have you calculated the reservation costs? There is no one answer to which is cheaper as ordinary tickets vary so much depending on when you book.

Eurostar is fixed cost interrail reservation €30 standard class or €38 1st class passholder in standard premier

There are limits on passholder reservations, they sell out before the trains are fully booked.

Times/availability and bookings can be made here https://www.b-europe.com/EN/Booking/Pass#TravelWish

French TGV also need reservations with passes, a limited number per train are €10, once they sell out that quota the price is €20 until the train is full, that is the same price in both classes but availability is per class.

link for domestic TGV booking here https://travel.b-europe.com/eurail-ge/en/booking-tgv#TravelWish

Theoretically you can travel as you please with interrail but with the need to reserve in certain countries (France being a prime example) that flexibility is limited. Local trains (eg TER in France) are still reservation free so it is possible to use without pre-booking.

There is no phone support for interrail.

It will also make a huge difference as to when in dec-it should be no rocket science that when Xmas (Noël in FR) nears, many more people go travel. Ohter as the tradition in GB prescribes, in FR there will still be trains on 25/12, but a kind of reduced sunday service. Also as about all EUR railways change their timings around 12/12-it will remain not possible to book for after that day till perhaps 2 month advance. €* is exempt from that.

You CAN do Paris-Avignon by TER=local/regional trains, RES-free, change Lyon, fixed 2-hourly service, it runs via what one now calls the ´classic lines´-which all trains used till this hi-speed TGV came. It will take about 7-8 hrs that way. vs about 3-3,5 in TGV.

YOu CAN in principle even do Wales-Marseille (=beyond Avignon) in 1 travelday: to LON, €*, the overnight train from Paris to Nice (will probably not run all days in dec.) till Msl (other station, Blancarde) , as in 1 travelday you can use as many trains as can manage-also overnite till alighting.

Then tour around day 2 and finish in Avignon-the old in-town gare sits along the citywallls, there is also a brandnew way out of town TGV halt.

As an alternative consider a flight from UK to Nice and take your four travel days for something more fulfilling rather than 2 intensive days out and 2 days back.

I am just on my last of 5 legs  flight to Sicily, syracuse to Salerno, then to Bologna.Leg 3 through alps to Nuremberg  leg 4 to Brussels then tomorrow to . 

11 days of great travel