Ok.... So as far as I remember the sestet changes the mood of the poem. In the octet part it's angry and loud and passionate. Everything makes a noise, the guns, the bells the choirs... It's chaotic and fast and as said, obviously the language (well to me at any rate) is angry.
In contrast the next six lines are sad and thoughtful and without any hope. You read them in a much slower way (did you read the poem outloud? Sounds silly but it helps sometimes). The words he's using seem longer so you have to slow when you read it which means you take in what he's saying more because it's not quite so 'dum, dum, dum' in its message but rather 'da dum, da dum, da dum'... If that makes any sense at all! The language is 'echoey' in a sense, the letter L draws out a word as does the haunting 'eyes' and 'goodbye' and 'minds' and 'blinds', personally I think there's alomst a tenderness to his rhyme in the sestet which isn't there in the octet. he only uses a full stop once before the end where he uses 2 before the end of the octet part... Again, hamering home a message rather than giving you time to slow down and take in the words and reflect.
I assume the use of a sonnet for the type of the poem he's writing you've already covered so I guess there's an element of the satiric in there too you've picked up on. The sonnet with a sestet is itallian in origin (I had to look that up!) and apparently Dante used them in his sonets which makes me wonder if that had anything to do with Owen's choice but that's just a thought knocking around in the back of my head, I have no idea whether it played any part.
Does any of what I said make sense or help at all or would you rather I focused on something else? Or give up completely! Lol