In Mark 12:6 both the Gramcord and Accordance databases parse ἔσχατον as masculine—I assume because it is parked next to the masculine form αὐτόν (the form is the same for both masc and neut acc sing). Yet syntactically it appears to function as an adverb, modifying ἀπέστειλεν. When adjectives function as adverbs they are usually neuter. If this were masculine, this would suggest, I think, understanding it as substantival in apposition to αὐτόν: “he sent him, the last one.”
I’d put it in English something like this:
“He still had one [to send], a beloved son. He finally sent him to them, saying ‘They will respect my son’.”
Does this analysis make sense? Or am I missing something here?
See BDAG, 397–98.2.b (end) for ἔσχατον as an adverb; for a discussion of adjectives that function as adverbs, see Wallace, 292–93.