I had similar struggles on my SC leading up to my exam and similar issues with my GMATPrep verbal score turning out much lower than my
MGMAT score. I actually pushed my exam back to get over that hurdle. I never really got comfortable with SC but I found that if I focused on the structure of the sentence, it helped me. Initially, I really slowed down my SC - I was spending a good 4-5 mins on each question to find every error I could. Then I started to speed things up. So for example, here's the run down I would do for each SC question as I read the sentence:
1) Identify subject / verb / object - check agreement
2) Identify any subordinate clauses / modifiers (ignore them for the most part - although make sure they modify the correct word)
3) Look for parallelism - make sure you know all the parallel markers and rules inside out
4) Look for pronouns and check your antecedents
5) I depended on my ear for verbs (I wouldn't recommend this for most though - it just seemed my ear worked better than over-analysis of the tenses)
That allowed me to get most of the easy and medium difficulty questions right. At that point, I still looked into the 700+ questions but accepted the fact I would get a few wrong. I spent the rest of my time bolstering up my quant score - I have a quant background so knew that I would get better bang for my buck on that side with the days leading up. You might find the same - improving your strengths may help get you a higher overall score.
As for the AWA - look at chineseburned's post. I didn't spend much time on the AWA. I used the template - got a perfect 6.0 and I'm no professional essayist! Try out GMATWrite if you want to - it gave me confidence that I was doing the right thing so at least that was one thing I didn't have to stress about during the exam.