Post History

Any understatement could be unintentional, or it could be motivated by pragmatic reasons such as hesitation to bring up a controversial point.  

However, more often than not, blatant or ridiculously formed understatements are used ironically; and the irony very often concerns the understated quantity itself, indicating an absurdly *high* degree of that quantity.

"Fulsome" (originally spelt "fulsom") *might* count as ridiculously formed[^1].  As you observed in your point #4, "full" expresses the absolute fullness, while the suffix "-some" indicates a degree, contradicting the fullness.  The absurdly high degree of that quantity could then be "over-full", "over-satiated", with negative connotations.  That meaning is given as one of the possible/historical meanings in your reference - which would mean that the irony would be getting conventionalized and lexicalized over a part of the word's history.  The hypothesized irony could thus explain the negative connotations behind the sense "over-satiated".

However, perhaps we don't need to invoke irony to explain why "fulsome" sometimes acquires positive connotations and other times negative connotations.  [This lexicographer's lament](https://www.merriam-webster.com/words-at-play/fulsome#:~:text=In%20the%20United%20States%2C%20the,an%20etymological%20root%20is%20incorrect.) offers an alternate possibility that "fulsome" would tend to have positive connotations when associated with "full", and negative connotations when associated with "foul"[^2].  We all learned to treat "fulsome" as a single word with several meanings, but its etymology could involve an uneasy merger of two homonyms.

[^1]: Personally, I don't find anything ridiculous about the modern expression "rather full", and I'm not convinced that mid-13th century "fulsom" should be perceived as ridiculously formed at all.

[^2]: In [Scottish English](https://www.merriam-webster.com/dictionary/foulsome), the spellings are (optionally) kept separate.