Barcelona to Figueres- train-

I believe you have to select a specific return time for the train when you buy a round-trip ticket.

Buying two separate tickets isn't horribly more costly than buying a round-trip--assuming both purchases happen on the same day. It appears that the difference between a round-trip ticket and two one-ways is about 9 euros. I see a lot of tickets available for tomorrow at the same price shown for November 6 (arbitrarily selected date), so I'm not sure Barcelona-Figueres fares ever change. If that is the case, you many not need to rush to buy tickets. The main thing is to buy AVANT rather than AVE tickets. Doing that will save you over 10 euros each way.

There doesn't seem to be much to see in Figueres beyond the Dali Theatre and Museum and the co-managed jewelry collection (uncrowded, free with the T&M ticket, very nearby and recommended). If you miss the bus from Figueres Vilafant into the center of town and there isn't a taxi lurking (I would expect to see one at the station), the walk will take you 20 to 30 minutes each way; it would only be prudent to allow for that walk.

If you like to walk through historic districts, add at least an hour for that. If you end up without time to do that, I wouldn't consider it a tragedy. You may be ready for lunch or a snack by the time you finish at the T&M even if you don't take a walk through Figureres. If you're early by Spanish standards, you won't have many options for a full sit-down meal, so take that into account. Otherwise, the meal time can be longer or shorter as necessary to get you back to the Figueres Vilafant in time for a pre-purchased return ticket.

For my (in-depth, slow) pace in art museums, the calculation would be something like this:

Walk to and from RR station: 1:00
T&M: 3:00
Jewelry: 0:30
Walk around: 1:00
Food: 1:00
Total time in Figueres: 6:30 (5:30 with no time to walk around)

That would be conservative for me, and most others would spend less time in the museum. Warning: I don't like surrealist art, so a real surrealism nut might want more than 3 hours in the museum. It can be crowded, so some time can be wasted revisiting areas when the mob has dispersed. There's space upstairs for special exhibitions; hard to know how much time you might want there, too.

I was told the jewelry collection could be visited before the museum.

You don't need to buy a return to go to Figueres (or Girona for that matter), while the outbound journey in the morning can be busy certain days -hence the advice to pre-book- the return is almost always available to purchase on-the-spot. In this manner, you can choose at what time to return.

Note that, in the worst-case scenario, should you not find a return ticket for the high-speed train, you could always take a slow train back to Barcelona. It'll take much longer (+2h) but it'll take you back. And for these, you don't pre-book, just get a ticket on the spot and jump on board. If anything, mind the schedules of both, fast trains and regular ones. If in Figueres there's a "small" hurdle though, as you'll need to depart from the Figueres-Vilafant (a mile away from the centre) for the high-speed trains, but from Figueres station, located in the centre, by the Gaudí museum, for the regular trains.