He's right, there isn't actually any standard pronunciation of Norwegian (But for some reason we have two standard written forms). Since it is built up on dialects, you don't have to use consonant mutation, because some dialects don't. However, the dialect known as East Norwegian (which accompasses several different dialects and sociolect) is generally seen as what could be considered standard Norwegian, and that dialect uses consonant mutation so I would advise you to learn it.
Norwegian has a pitch accent system, yes. https://en.wikipedia.org/wiki/Norwegian_phonology#Accent & https://en.wikipedia.org/wiki/Pitch_accent#Norwegian_and_Swedish
Det . Slightly silent t. All the answers above seem very academic, do we have to study more, than just trying to learn the words and hear people saying them to be able to speak Norwegian. Iv been trying from time to time to learn for years, without much success. So much, that i want to leave, but have a daughter here. Id also prefer to learn the translation with the wrong English grammar, as most English understand and sometimes speak with the wrong grammar, its just confusing on this app , that you get a wrong answer if write wrong grammar in English. Yet everyone would understand the way the words are said in Norwegian, directly translate to English, without mixing them around. So Duolingo could be more flexible with English talk. ,