About /ʃ/ and /ʂ/
Disclaimer: I do not personally speak Norwegian. However, I know a bunch of guys who do and I asked them some questions. The topic was the exact pronunciations of ⟨sk, sj, skj⟩ and ⟨rs⟩. According to them, those are different, and not exactly as we are told about in the Basics 1 pronunciation cheat sheet.
⟨rs⟩ is a retroflex. That means, it’s not exactly a sh-sound, but rather a similar sound, but with the tongue curled backwards. Some dialects feature more retroflexes than just that, but they all occur when an r is present. ⟨sk, sj, skj⟩ however, are not retroflexes, but just normal post-alveolar fricatives. Your normal sh-sound.
In the Basics 1 table, you transcribe both as /ʂ/ and call that a sh-sound. The sh-sound is written in IPA as /ʃ/, while /ʂ/ is the symbol for the retroflex. Could you please correct your table as you see fit?
"rs" is pronounced /ʃ/ as in norsk, except when two words are joined together, e.g. motorsykkel, where it can be pronounced as /r/ followed by /s/, but many pronounce it /ʃ/ there as well.
I've corrected the IPA to /ʃ/.
Not all dialects have retroflexes. You will find them in the combinations "rt", "rd" (but not always), "rn" and "rl" as /ṭ/, /ḍ/, /ṇ/ and /ḷ/.
I disagree. "rs" as in norsk is pronounced IPA /ʂ/ with the tongue slightly curled back. This is not exactly the same sound as "sj, skj, sk" as in skjorte or ski which would be the IPA /ʃ/. The difference might be minor and probably varies among different speakers however