The marking scheme is out of 100. The allocate 30 marks for each language,purpose and coherence and then 10 for mechanics. Your misinterpretation may cost you marks for coherence and purpose, not sure how many though! If your language and mechanics are good you may still be able to get a decent grade! Don't worry about it now anyway because you still have paper 2 tomorrow which is an opportunity to bring yourself back up!
For the comparative make sure to read the Q's and pick the one that suits you the best. Firstly name your three texts and the authors/directors of them. Then address the statement in the question directly and come up with 2 major points from that. The compare those 2 points with each of the 3 texts, highlighting similarities/differences between the 3 !
I don't think you'd lose marks for not putting in the date on the blog, maybe mechanics could go down a bit but if that was your only mistake then I doubt it.