Nasalized vowels can lean toward close or open vowels. Anyone who speaks French should know that French nasalization favours open vowels to some extent, and the close vowels are shifted downward when nasalized (cf. ‹i› /i/ vs. ‹in› /ɛ̃/, ‹ue› /y/ vs. ‹un› /œ̃/, ‹o› /o/ vs. ‹on› /ɔ̃/). Since the two vowels in the quote above shift down when nasalized, it doesn't seem particularly unusual. Perhaps a better question to ask is, why does nasalized /u/ drop more than nasalized /o/, and why does it lose roundness while nasalized /o/ does not?why [do] nasalised /e/ and /u/ become /ɛ̃/ and /ʌ̃/?
Let's take a look at some tonal languages and see if this assertion holds true.You either use tones on vowels, or you don't at all. What I mean is there is no single language out there that marks tones on specific vowels and not on all, especially not with a contour tone system like the one you chose.
Mandarin Chinese has the four main tones, plus a fifth "tone", the neutral tone, which is essentially a lack of tone. Mandarin Chinese has strong and weak syllables, and while the strong syllables always carry one of the four main tones, the weak syllables tend to have the neutral non-tone. The tonic realization of the neutral tone is simply an extension of the previous syllable's tone. Others can describe it much better than I can, but the fact is that Mandarin Chinese, the most famous tonal language, has exactly what you are claiming no tonal language has.
Zulu also evidently has something similar, a low tone which is essentially a lack of tone into which a preceding high tone can spread.
Moving on, Cantonese has six tones, all of which may appear in open syllables (non-checked tones), and only three of which may appear in closed syllables (checked tones). With a little bit of sound change, it is easily conceivable that the checked tones could be reduced in number from three to one, or none, like Mandarin.
Several Southeast Asian languages with sesquisyllables carry tone only on the major syllable, while the minor syllable may be toneless or have phonetically predictable tone.
Serbo-Croatian has both rising and falling pitches, but these may only occur on stressed syllables; unstressed syllables lack this distinction.
Many examples of this.