My first attempt - 2 weeks of studying

My background:
Automotive/Aviation industry
Engineer: bachelors and masters (3.9GPA)
Non-native speaker

GMAT Study:
~2 weeks. English is weak point, so I spent most of my study time reading questions on gmatclub. There was not method to my studying.
For math, I spent most of my free time at work going through all possible rules. Whenever time permitted, I would attempt random questions on gmatclub.
Honestly, I am not a good test taker - nervousness gets the better of me, so my first attempt was just to gauge the exam, first hand, and see where I stand.

Mood before the exam: Slept at 2am, the previous night, for ~6 hrs. I did have a mild headache, but 2 aleve pills did the trick.

Exam sequence: Verbal, Quant, IR, AWA.

Verbal: My 5th question was a bold face and then some pretty tough CR questions. I am sure I messed up most of the CR questions. Surprisingly, I was pretty efficient with reading the RC passages. At the end I couldn't complete 3 questions.

Quant: Although, and again, I couldn't complete 3 questions, I think I did pretty decent. There were a 2 or 3 DS questions on which I spent more time than I should have, making each of them a scientific experiment. I didn't get a single probability question.....well maybe the 3 questions that I missed could have been probability questions.

Score:
640 Q47 V30

Next Steps:
Give the GMAT in 4weeks [hopefully in 3 weeks].
Focus primarily on verbal, specifically CR: start with the Powerscore CR book.

Advice needed:
Since I am a non-native speaker, I need help with a good source to start my SC study - from scratch; A source that will, at least, give me a good handle on written grammar rules. Although the forum is the best place to learn from, I have often fond my self lost with many SC explanations; I, then, find myself spending more time, than i should, researching specific grammar rules.
For example: In an OG SC question which used "Although," one of the explanations stated that in the construction Although[X], [Y], both X&Y need to be IC's. But a few days before my GMAT exam I came across another SC questions that used 'Although,' however, this time, one of the explanations stated all the convoluted ways you could use 'although.'
This brings me to another question: There are so many idioms and rules in the English language, but do we really need to know every possible idiom and rule (and the exceptions to each rule) to do well in SC?

Finally, I often find it tough to gauge what a 95% hard questions really means on gmatclub. I spent a lot of my time trying to solve 65%+ verbal questions on the forum. However, just 3 days before the exam, I realized my blunder. When i got to the 45% level questions, more realistic for my level, I could barely get 70% accuracy.
For my next attempt, if I manage to hold my quant score at ~47-48. What level verbal questions should I focus on to improve my score by, say, 60-70 points?

Thanks in advance!