Read the reservation advice pages on this site. SNCB can normally do these or ring SNCF.

ive managed to sort out the reservations from paris to latour, but the spain part isnt available yet - does anyone know when is the earliest in advance u can book reservations for barca-malaga?

Booking Spanish trains outside Spain is either not possible or very difficult via phone. The advice is to go to any ticket office in Spain with your pass.

There are many trains from Barcelona to Madrid and also regular trains to Malaga from Madrid. I believe there are a couple of direct (but stiĺ via Madrid).

No-there are 2 daily really LONG-dist thru trains Bcn-Andalucia (with various destinations-they split in Cordoba) and these are often seen as THE bottleneck to get on-somehow I also kind of suspect RENFE does not want most seats taken by pax who hardly pay. These do NOT run/stop via Mad-and hence the trip time is far shorter (over 1 hr) as via Mad+change.

The usual service Bcn-Mad is hourly, Mad-Mlg 2-hourly (hourly to Sevilla)-but that would be extremely easy to see on renfe.com

Here are the known ways to book Spanish domestic trains: