for the poem part, i think that translating a poem is always a tricky part so i still struggle to understand it fully...but for me, how i see it, its thats sajou completed and he was willing to complete the part that kusakabe forgot. because when hara (the professor) forgot the rest of the poem or said the wrong prose, sajou didnt respond or complete. so, it may be just my interpretation, but i think its just a proof of sajou and kusakabe complete each other really well and accept each other... i got it when i watch the movie honestly lol and it still MY interpretation so i may be very wrong too lol
for the "pedo" part, i think its partly linked about history of yaoi/bl/shonnen ai (wtv you like to call it). it "glossed over" so easily because it was a pretty popular trope at the time (the manga is beginning to be kinda old now)..if you look back at the history of bl, it always tackled with harsh themes with more or less subtlety and all...i think on of the reasons why the way harada is super creepy is not really acknowledge during all the manga and is even giving a full side story :/ but again i might be wrong!
(also english is not my first langage but i hope you could understand everything and that i could answer clearly everything!!)

i will skip creepy harada but really im in love with asumiko nakamura draws and tells her stories but sajou and kusakabe hold a special place in my heart i love them and watching them grow together...
i have the two volumes of blanc at home but i think i will purchase all the series to reread it in my language too (=・ω・=)