Janus Bahs Jacquet left a comment pointing out that νῶτον exists as an independent word in Greek (a neuter noun) with the meaning "back". See the linked LSJ entry for further details. A synonymous variant masculine form νῶτος also exists.
I think it's not really any more accurate to call -notum/-notus a suffix than it is to call -arm a suffix in the word forearm. And I think it's unnecessary to call -notum/-notus a "combining form" because it's really the same form as the independent word. A special combining form of νῶτον exists for use in words where it is not the last element of the compound: that combining form is νωτο- (noto-) (or sometimes not-, before a vowel), as in your examples of noto-chord and not-odont.
The occurrence of "Pronotum" in Burmeister's entomological texts
According to the Oxford English Dictionary (OED), pronotum was formed in German (as Pronotum) by Hermann Burmeister, first appearing in his Handbuch der Entomologie (1832) alongside the parallel formations Mesonotum and Metanotum. The OED identifies the last element as Ancient Greek νῶτον, discussed above.
Unfortunately, I haven't been able to view the German source text (if you think you'd have better luck finding the cited passage, the OED specifically cites "I. i. iii. 81" as the location in the 1832 handbook where the word Pronotum occurs).
I found an 1836 translation of Burmeister, A manual of entomology, (translated by W. E. Shuckard), that gives a etymology that doesn't seem to agree with the OED's:
The superior, which we call PRONOTUM* (Pl. IX. and XII. A,A,A, Prothorax of Kirby and Spence), takes very different figures. [...]
*This name is compounded of προ, anterior, and νότος, the back.
(p. 75)
The title page says that this translation contains "original notes and plates by the translator", so my guess is that Shuckard misspelled νῶτος as νότος in this note. Both Greek spellings would be pronounced the same way in the traditional English pronunciation of Latin and Greek (and also in modern Greek pronunciation, actually), but νότος seems to be a different word meaning "south wind". Despite the misspelling, and Shuckard's reference to the masculine rather than the neuter variant of the Ancient Greek word for "back", the meaning given by Shuckard is consistent with the OED's explanation.
Latin notus and English "notable" aren't relevant
Latin nōtus "known" is unrelated, as shown by the etymologies that you cite: PIE *ǵneh₃- is different from PIE *not-.
And "notable", from Latin nŏta, probably is unrelated to either of the preceding words. A traditional etymology connected nŏta with nōtus, but the short vowel in nŏta makes that hypothesis difficult to support: de Vaan 2008 writes that "Schrijver 1991 has clearly shown that it is impossible to derive nota from [...] *ǵneh₃- 'to know'" (Etymological Dictionary of Latin and the other Italic Languages, p. 414).
I don't see any way pronotum could represent "the notable supporting structure": it wouldn't be put together the right way (assuming you're thinking that pro- might provide the meaning "supporting" and -notum might provide the meaning "notable") and the parts don't mean that.
