• "I can see those trains on SNCB with a normal search, but not when I ook with an Interrail pass.” => That usually means the pass holder seats are sold out. For which date is this?
• Spanish railways don't send their complete timetables to the rest of the world, so any planner will be very incomplete, except for RENFE's own one.
• Euromed trains can be used with Interrail. “SNCB told me that I can’t include this train’s company on my pass” - did they say why?” Is the train full maybe? For which date? By the way, there are many more trains than this one between Barcelona and Valencia, please check the RENFE website. SNCB can also book them, according to their info. They will probably need the train number since their planner won't be complete.
• For ALVIA trains or any other trains in Spain, check the RENFE website. To add them to your trip, you can do that manually if they're missing in the rail planner app. If trains are not in the app, you can still use them.
• For your return, what trains exactly do you want to use on which date?
• Since you seem to live in Belgium, all trains you mention are best booked at the Belgian railways ticket office (one that can do international reservations). They can do Interrail reservations for France, Spain and international trains to France and Spain, without the extra booking fees and postage fees that Interrail charges.
• Reservations for Spanish domestic trains cannot be booked online. They can be booked at the station in Spain though (and in some other countries). Since Interrail reservations for Spanish domestic trains can be booked until the train is full, it's usually enough to book a few days in advance.

Yes, RENFE are a pain. And SNCF and Thalys too, to a somewhat lesser entent.

Brussel-France with Thalys or TGV you should get the reservation via interrail.eu or SNCB ticket office (international counter) or via SNCB International call center (for Thalys also via SNCB International Website - Interrail Reservation Thalys/Eurostar). 

France-Barcelona should be able to sell SNCB International counters and call center, interrail.eu can only sell it as printed ticket via mail. You can also try at SNCF counter or DB counter or Call Center (but delivered by mail). 

Trains in Spain are not bookable via interrail.eu, but Interrail is valid in trains of RENFE (except AVLO and EVO). You can call RENFE call center and buy via phone the reservation and collect them at ticket machines in Spain. In Spain a lot of trains miss in the interrail timetable because Renfe didn’t send them. You can check the timetables at Renfe website. 

Hello!

I read on the forum that SNCB could book international trains, but I asked at the station and for once, the solder was very aware because he did an Interrail in Spain himself. Yes, it's written on the website that they can make reservation for lot of trains but it os no more the case. They lost this ability a month ago.

So, I will come back today and ask for the Thalys, but it's really not full. It is for the 2 of April (I know is near Easter but I'm a teacher so I have no other date), but when the train is almost full, prices are very high. I check lot of other dates, just to see and it seems that some trains are just not possible for a pass holder, even in more than a month (maybe because they know that the train will ne full, even if it is clearly not full yet).

Actually, I'm not sure that after all those fees and calls, the pass will worth. Trains are not that expensive in Spain, Ouigo is almost free in France, and the Izy costs 15€. I did two interrail in Italy and it was really nice, but here it is too complicated and expensive.

Thanks for the information. I'll ask SNCB customer support about this.

German Railways can still book domestic reservations in Spain (confirmed last week), at the ticket office or by phone (+49 30 2970). They need to know the train number since their timetables are also incomplete for Spain for reasons mentioned earlier. They can send reservations by post for EUR 5.90.

That is very well possible.

Italy can also be a pain, but at least booking reservations is less complicated.

You basically summed up the reasons why certain railways are making Interrail travel complicated and expensive. I suspect some of them do it intentionally, unfortunately.

@rian: I was in ES some 2 weeks ago-Andalucia and noted that RENFE now has completely different ticketing for MD (and probably also Talgo and IC on the old broad gauge) as for AVE/Avant (these are still the SNCF-style old airline tickets). Ticketing for cercanias is also much more complicated, as they now want you to have an additional Renfe-esTU customer chipcard with any form of ABO (I bought a new Tarjeta Dorada=senior discount). This may explain or support this.

Besides: AVE trains are quite expensive in ES- even a 1h20m sector like Cordoba-Malaga is around 35€ now!

35 EUR for a 2nd class Interrail reservation? Ouch.

For Cercanías, we'll see. Maybe I'll ask RENFE customer service what you're supposed to do. Cercanías have always been included until now.

I now have confirmation from Belgian railways (NMBS/SNCB) that they recently decided to stop selling ATB tickets (the classical international tickets on paper) and as a consequence, they don't sell any tickets or reservations anymore for international trains to Spain or domestic trains in Spain. Tickets to/in Spain are available as PDF tickets/etickets (as we can see on the SNCF website or TheTrainLine.com) but apparently, they don't have a possibility yet to sell those.

On the other hand, all reservations that they do sell, are now available as "homeprint”, so they can be sent via email after booking by phone. This includes reservations for Italy and many TGV reservations (except to Spain).

I have written an e-mail to the ‘Junta Arbitral Nacional de Consumo’ in Spain in order to file a complaint against Renfe for their constant invalidation of Interrail passes. Maybe someone wants to do the same or has a better idea as to which entity might be able to help. Here’s the mail address I have used: 

Muy bien! Gracias! Hopes it helps.

Or perhaps if not-the ever increasing competition on the rails in ESpana itself-or their funny ambitions to even try themselves foreign ventures-if they do that the spanish way doom+failure is certain