gave good heed - literally, “he weighed.” The “teaching the people” seems to have been oral; the “proverbs,” in writing. There must then have been auditories assembled to hear the inspired wisdom of the Preacher. See on the explanation of Koheleth in the Introduction, and Ecc 1:1-18 (1Ki 4:34).
that which is written, etc. - rather, (he sought) “to write down uprightly (or, ‘aright’) words of truth” [Holden and Weiss]. “Acceptable” means an agreeable style; “uprightly ... truth,” correct sentiment.