Thank you very much! :)
First: forvo.com is really good, I didn't know about this website up until now and it's really great to mimic pronunciation.
I don't know if I'm wrong, but I hear a difference between the "ll" in "fjall" and in "illur". It does indeed sound like a voiceless L-Sound in "fjall", while it sounds approximately like the english L in "illur" to me.
I prefer to mimic pronunciation as well and my knowledge about linguistic terms is rather low. A glottal stop however is just the sound you make in English too, when you try to stop speaking abruptly. Like in "Uh-oh" for example. I was asking because "vatn" sounds to me exactly like that. As if someone is saying "vat.." and instead of an "n" the speaker suddenly stops.
But knowing about forvo.com now, I can just mimic pronunciation as I usually do, instead of trying to learn the rules, thank you.