Anyway: Is my perception right? I mean, is *ngan* when being part of the name of an inhabitant mostly not stressed? And what about the "number-forming elements", they are also usually not stressed, right?

Going by Okrand's pronunciation and a complete lack of any canonical counterexamples anywhere, I believe you are correct.

(I can think of a non-canonical counterexample. In TNG's "Since of the Father," the characters consistently give stress to the second syllable of cha'DIch. They also pronounce it more like cha-DEESH.)

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SuStel
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