Translation:I think I expressed myself very well.
Similarly, "I think that I have expressed myself most well" (instead of "very well") was rejected by DL on 2 October 2020. Why? The only reason I can think of is that "very" is indeterminate and only functions to add emphasis, whereas "most" converts the translation to a statistical claim that my expression was better than that of all members of a particular set, including the set of all prior times that I expressed myself. But that distinction might reasonably be classified as nitpicking, so I'm at a loss to really understand DL's rejection.
Yes to "espressa". When essere is the auxiliary verb, the past participle changes with gender and with singular/plural (sono andato/sono andata, siamo andati/siamo andate). I think "benissimo" would stay the same because it's an adverb here, not an adjective, but I'm not 100% certain on that.
I realise I'm an idiot, but I put "I think they expressed me very well" - for some reason I was imagining it was a painting they were talking about or something. It wasn't accepted obviously, but I'm not actually sure why not. Is it because it would revert back to 'hanno' when talking about others?