You can raise your score by 70 points, especially since it sounds like you have some time before you sit for the exam. You give a good analysis of what happened, especially missing 8 or the last 12 questions. If you could correct that portion so that you miss one or two instead of 8 that should take you where you want to be.
Here are some specific ideas:
As I told one of my tutoring students the other day, for most people, Sentence Correction is the one place where you can go more quickly than the average on Verbal. In other words, working quickly on Sentence Correction puts time in the bank that you can then withdraw for harder CR and RC passages. This is where you may want to start. Work on identifying the key differences between answer choices. When you find an error quickly check the other choices for that error. Eliminating multiple answer choices at one time is one key to both accuracy and speed on sentence correction. Also, make sure you are focusing on substantive grammar differences as opposed to style difference. For example, try to find a difference like singular vs plural, as opposed to something more stylistic like "maybe vs probably."
And then when you have a problem down to two or three choices and obvious grammar errors on gone, remember that the GMAT writers' favorite way to be "clever" is parallelism. Something like, "In 1980 U.S. companies produced more cars than they produced in 2008." See the use of both "produced" and "in" twice. The first time I took the GMAT four years ago, I did not focus on parallelism and I missed about 4 sentence correction out of 14. When I took the test again recently it was rare that I would miss a question in practice. I hope this work as well for you!
As to critical reasoning, you may find that PowerScore is not specifically tailored to the kinds of questions that are on the actual GMAT. Quick recognition of question types and having set strategies for those types is essential for Critical Reasoning. If you are thinking of a dozen different types of questions you might be over complicating what is found on the GMAT.
At Veritas Prep we use the SWIMMER system to categorize questions and then apply the appropriate technique. Also please remember some simple things about critical reasoning, such as the fact that when you strengthen or weaken an argument it it is the conclusion that you strengthen or weaken. Are you able to quickly identify the conclusion when you read an argument? This is crucial. If you are looking for some specific GMAT focus on CR as well as lots of practice questions you might check out the Veritas Critical Reasoning 1 and 2 books. I mention them because until last month they were not for sale except as part of a Veritas class, but now you can get them as a stand alone at Amazon.
Hope this helps!