1. Why could I not solve the difficult Quant questions on this test (got 7 wrong in a row) despite solving the difficult questions in OG 12 pretty easily.
OG questions are around 650 level. Very few questions are there at 700 level. Use OG questions to review a lot. You should know all the methods to solve a single problem. Say for slope problem, you should be able to approach it in algebraic form also you should solve it using diagrams. Choosing the best out of two depends on your ability. Choose any method which gives you maximum accuracy and minimum time.
2. Why did my scaled score go to 32 in verbal; I got 4 wrong in first 20 questions and I was told in many guides that its the score at start that matter most. Though I got 10 wrong out of last 17.
If you are spending too much time for questions in the beginning, then you have less time to answer the last few questions. Spend time equally. The first few questions matter, because if you make many careless mistakes in the beginning itself, then you will have tough time reaching the actual level you are capable of. So first few questions are stressed to warn that you should not make careless mistakes, you should be more cautious and it doesn't mean that you should spend more time to solve them. Your focus during preparation should be on reducing careless mistakes to help you get your maximum ability gauged by the test. Scaled score depends on the overall performance. Improving your content knowledge is the best way to perform well. The difference between a 750 level test taker and a 650 level test taker is that a 750 level test taker will be able to solve the first 10 questions easily and quickly, while a 650 level test may be able to solve them with more time, but at the end you will be where you started because with less time and more difficult questions to answer at the end you will eventually fail. So its all about content knowledge and not about how you manage your time. Follow a timing strategy and don't set any priority to any part of the test.
Also can you plz suggest me what to study for 50V and 40Q; my basics are alright but I am willing to start from scratch specially quantitative. I have 1 month left in exam.
Review extensively as said above. Reduce mistakes. Take mocks once a week, find your weak areas and hone them. Get better in concepts.