mysterygirl,
If you are beginning to falter at the 28 - 30 question mark in the Verbal section, that might indicate that you need to work on your stamina. One simple way to improve that is to do less work on the front end of the test. Are you utilizing a template for the AWA or are you knocking out the essay from scratch? If it is the latter, employing the template can help you minimize the brain power expended on the AWA portion. Let me know if you would like a good sample template.
Regarding your SC challenges, I am attaching a very good document that covers the Parallelism rules. I like this document primarily because it sets things out visually, but also it is concise. There is a good rule of thumb to use for modifiers - that is, that they should be as close as possible to the noun that they are describing. However, there is one exception to this rule and it is also good to know (and look for because the GMAT likes to test this exception on the more difficult SC questions) - participial phrases (a clause beginning with an -ing verb) can modify a subject that is not immediately preceding it, if it is at the end of the sentence and set off with a comma.
Think of Assumption questions like Strengthen questions. They are more difficult because instead of the correct answer directly strengthening the argument, it actually removes a flaw (that you might not have even considered) that, if true, would be problematic for the argument. There is a technique called the Assumption Negation technique that many use on assumption questions. This involves taking the opposite of the answer choice consider its impact on the argument. The opposite of the correct answer choice will void or undermine the conclusion of the argument. Many people find this technique difficult and I tend to use it only when I cannot determine the correct answer or to choose between 2 or 3 remaining contenders.
Finally, I think you might be overthing the Main Point questions. The answers are generally simple (e.g., to tell us about a new theory of how butterfly nebula were created. or to explain the problems of a current program and recommend ways to improve it) but they always include the topic of the passage and what the author is saying about that topic. Many attractive but incorrect answers might include these two elements, but may have the scope of the topic too broad or too narrow, or they may not capture the author's tone on the topic correctly. So, as you initially read the passage, read it not for the details but for: 1) the topic and scope; 2) what the author is saying about the topic (is it informational, analyzing something, or simply an opinion on the topic); 3) how the passage is organized (chronologically, point/counter-point, etc.); and, 4) why did the author write it.
Hopefully these tips help. If you have any questions or want additional information, let me know.