Very lost about seat reservations

Hi!

I’m trying to plan my trip, but unfortunately the seat reservations system is a nightmare.

I read the guides here, even the one from The man on seat 61, but it’s still not clear.

I want to travel from Barcelona to Heidelberg on 2/9. There are several options, I would like to use the one that is 10:33 → 22:53. But I’m flexible, so I can change that to any of the other 3 options that arrive the same day.

I’m trying to make a reservation at Interrail.eu .

But all the options have an issue.

1. The one I mentioned, doesn’t allow to book the train Paris Est - Mannheim. I read over here that this should be made by phone and they send it via post mail to your address? Will it arrive on time?
2. Another option is 08:22 → 21:49. In this case, I can’t book Barcelona - Lyon, should I go to the train station and directly book there? But then in the same journey, I can’t book Paris - Karlsruhe. So what happens if I book Barcelona Lyon, only to find out that Paris - Karlsruhe is sold out? How do you go about this very intrincate entanglement of reservations on-site, by phone, via internet, etc?
3. The third option I’d like to use, 10:33 → 22:49, apparently allows me to make the reservation directly at Interrail.eu, but then when trying to add them to the cart, “Something went wrong. It looks like we can't add this journey to your cart right now. Please try again later.”. So again, I get lost on how to approach this.

Can you clarify a little, please? :) I’m about to ask for a refund and book a flight, I wasn’t expected this much hard work and stress just to be able to use my pass.

Edit: I’d like to add that on top of that, I read that for some reservations you need a pass cover number (???), whatever that is, and when I go to the link provided to get that, I get a “page not found” error. So I don’t know when, how, and where I need this extra number. Shouldn’t my pass be enough?

Thank you very much!

Have a nice day.

Tonga.