That's ridiculous. If you're concerned about "emphasizing size over strength" then why the hell are you doing sets of 12? And if putting the bar down tears up your hands then you are obviously doing it wrong.
No wonder you didn't get 455x5, why would you go for max reps on a lighter weight before your heaviest set of the day? Your technique (other than dropping the bar) is fine, but there are some issues with the way you train.