You should be able to buy Barcelona - (Madrid-Atocha) - Sevilla on eurail.com.

On 2nd August you'll have to travel via Madrid (2 x 12€).

On 6th August the direct train from Sevilla is available : 16:07 - 21:50. Otherwise via Madrid (2 x 12€). Book ASAP.

The Sevilla - Cadiz trains aren't available on Interrail/Eurail it seems. Buy them when you arrive in Spain, at any train station.

Thank you for your answer, but I more or less did this but there are already hardly any trains available anymore between Cadiz and Sevilla if I look on the Renfe online timetable. Is there an alternative without a reservation needed?

Yeah but RENFE is weird and does the same thing as other operators (SNCF mainly...).

If you try to book Madrid - Sevilla - Cadiz you'll discover that all MD trains are available for the last leg. Same for the return. They knowingly prevent the sale of Sevilla - Cadiz only tickets in favour of more expensive tickets from Madrid. (I'm not sure if that's even legal but that's another topic.)

That's why I'm confident that you can get Sevilla - Cadiz reservations at a train station. Does someone have more info on that ?

There's no other alternative that I know, even regional trains require seat reservations in Spain...

oh thanks, I did not know regional trains require seat reservations too

Yeah Spain isn't exactly pass friendly…

Worst case : I'm sure there are frequent buses between Sevilla and Cadiz. But best book the long-distance AVEs well in advance.