how badly would i be penalised for not concluding my language analysis? i had a comparative SAC today and managed to write everything but the conclusion. would the quality of my piece be more important than whether or not i finished? really didn't want to drop a single mark in english... oh well 
Not at all, unless your teacher is a real stickler for essay structure. Even then, the quality of your analysis is where, like, 98% of the marks are. Little things like the intro + concl. are just there to make a good first and last impression, but if your analysis was good enough not to need that last impression, you'd be fine.
Not having a conclusion is unlikely to cost you a whole mark on its own (esp. in the exam when it's marked out of 10, but even for your SAC where it's out of 20) but it might feed into the marking scheme somehow.
So if you were lacking a conclusion
AND your paragraph structure was imbalanced and hard to follow
AND there was a bit of repetition in your analysis
AND some word choice issues,
then you might lose a mark or two.
I wouldn't stress about losing a mark even if that does end up happening though. It's great that you've set your standards so high, but don't let a 19/20 discourage you

URGENT. Have an English SAC tomorrow. I swore I'd get an early night tonight but instead I'm cramming as much info as I can before my self-imposed bedtime.
What's the technique called when someone does something like this:
If we do this, A will happen. If A happens B will happen. If B happens then C will happen.
Where like you're sorta being driven down this path of logic and you aren't really left with room to go like "If A happens, D could happen instead of B"
What's that technique called?
I've just been calling it appealing to reason and logic which is wrong.
I'd call it
leading logic or
cumulative logic. Technically, the formal name for it would be the slippery slope fallacy, but calling it a 'fallacy' sounds a bit evaluative. Most teachers probably wouldn't mind, but I could understand some taking issue with you using words that are critiquing their arguments instead of metalanguage that discusses
how the author is being persuasive.