No idea about the different train numbers. In any case, reservation is optional for the train Ljubljana - Zagreb. Some source say departure is at 8:25 so get there early enough.

The reservation Zagreb - Split is best done in Croatia, e.g. when you arrive in Zagreb. Price is around 1€. Interrail can book it (sometimes...) but then charge 3€ + 2€ booking fee + 9€ postage fee.

Thank you very much for responding so quickly indeed. I have a few last short questions:

1. In the first place, if I have the pass purchased and activated with all the tickets and the QR of each trip that I am going to make, does it mean that I am going to get on those reserved trains, yes or no? In other words, they can't take away my seat if I don't reserve a seat, like on the Ljubliana-Zagreb train?
2. So I shouldn't reserve any seats until I'm at the Zagreb station?

Not sure what you mean but if a train does not require reservations then you can just board after having added the train to your pass. You will then have a QR code, which is your ticket for the day.

It worked well for us last August. Reservation in Ljubljana was not possible for the train to Split so we did it on arrival in Zagreb. The train was about half full. Not sure if it can be booked elsewhere. Maybe via DB or ÖBB - but then you still have the problem of collecting the reservation.

Ok, it's just that I'm worried that they won't let me get on the train from ljubliana because I don't have a reserved seat. Is that possible? I mean, what worries me is not being able to get on the train because it's full. If I already have the reservation with the QR of that trip, they can't prohibit me from going up, right?

And another question, 

As you can see in the two images, for the same train it says a different departure time on the web than in the application, is this normal? Do I make a seat reservation?

Thank you!

We used these same trains last year in August. Without a reservation for the first train (we checked the departure boards on the day before, just to be sure, but there was no mention of a mandatory reservation - the app was wrong). It was just fine. If you want, you can book a reservation in Ljubljana when you arrive there. Should be only a few euros.

If you have a reservation, indeed they can't stop you from travelling. If there are more people than seats, then some people will have to stand. That's how it works with optional reservations.

Please update your app. I have the latest version (23.3.0) and it shows the updated departure of 10:56.

Generally though, when in doubt, always check the national planner. Czech railways say 10:56. The rail planner app comes last in terms of reliability: it is only updated about once per month and should therefore not be used to plan. For international planning, it's better to use the DB planner or DB Navigator app. It has the about same coverage as the rail planner app but is updated twice per week.

This train Prague - Budapest has optional reservations. If you want one, it's best to book via Czech railways (76 CZK, about 3€). Look for the connection and then click on "I want a reservation only”. But I think it will be fine.

Thank you very much!!!!!