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.

Ok, I'm not any expert, just an intermediate learner. Here are my thoughts: 1. As far as I know yes 2. -ll is the same everywhere, whether "illur" or "fjall". Check: http://forvo.com/word/illur/#is and http://forvo.com/word/fjall/ 2.1. As for -nn, it depends on the surrounding letters. -NN in "hann" or "brennur", for instance, is pronounced as a long "n", while "nn" in "steinn" or "Spánn" is pronounced like "-tn", similar to "vatn". I don't know much about linguistic terms, e.g. glottal stop. I prefer to listen, compare and mimic instead.