I only see that canon examples sometimes use "must" and sometimes "need". Maybe it's me again proving my lack of english knowledge that you see more than I do? Or is this up to personal interpretation?

Here's an excellent video explanation of the difference between need and have to (and have to equals must). It says that have to focuses on the consequences of not doing something, while need focuses on the benefit of doing something. If you have to do something, there is some agency requiring it of you; this is not the case with need.

However, the reality isn't all that clear-cut. People interchange need, must, and have to all the time.

For instance, while nIteb SuvnIS DevwI' is translated with must instead of need, there is no sense in this sentence that any agency is imposing lone-fighting on a leader; rather, it's saying that a leader has a personal necessity of lone-fighting. So don't count too much on these terms having distinct meanings when used in colloquial English.

Now, if someone wants to pull up a few instances of -nIS being used where an agency is imposing an action and consequences of failure on the subject, and not just instances where the English translation uses must, I'll be happy to revise my opinion.

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SuStel
http://trimboli.name