Advance tickets will likely be cheaper as Interrail requires extra reservations on high-speed trains. The cross-border TGV is particularly expensive.
Maybe for a return journey it'd be better value but I'm not sure.
According to current timetables (which shouldn't change much) Barcelona - Marne-la-Vallée is best done like this :
- AVE Barcelona-Sants - Lyon Part-Dieu 08:19 - 13:20
- TGV Lyon Part-Dieu - Marne-la-Vallée TGV 14:00 - 15:46
The earlier you buy the tickets the cheaper it will be.
EDIT : it's possible to do Alicante - Paris in a long travel day but I'd suggest splitting the journey (like you're planning to) or taking a night train from the border.
Thank you, I'll have another look.
Do you happen to know how far in advance tickets are available? Where is the best place to get tickets?
The AVE to Lyon will be on renfe.com. They release dates in batches, you can't really guess other than check regularly.
TGVs from Barcelona or Lyon will be on sncf-connect.com. Have a look at https://www.sncf-connect.com/aide/l-ouverture-des-ventes