I had almost the exact same problem. When I did the diagenostics exam for Kaplan, the main reason behind my real low verbal score was the sentence correction and RC. My CR was fairly OKAY. But I almost got every single SC problem wrong. While i'm not a native English speaker, I went to college in the US; so I would expect my english proffeciency to be decent.
As a starting point for the SC, I read alot about SC and figured out that most high-verbal scorers tend to master the SC part of the verbal and leave some room for RC or CR mistakes and still end up with 44+ score on the verbal section of the GMAT. Furthermore, it turns out that SC is in fact the easiest part of the GMAT to improve on. I couldn't agree more, as most GMATTERS kept emphasizing over and over about how easy and important to master the SC with little effort and short time compared to CR and RC.
Materials I used to start working on my SC skills:- Spidey Notes
-
Manhattan GMAT SC book [I know you've read alot about how good it is; well, it really is great.]
- OG 11 and OG verbal review books
How to approach it:
started reading the
MGMAT SC book and spidey notes and write down the main weak areas to me. I ended up with about 5 sheets, not to mention my huge problem with Idioms as most non-native english speakers suffer from. Afterwards you start solving problems from OG 11 and OG verbal to test your comprehension and understanding of the SC rules and concepts. As you solve more and more problems, you will start to notice a pattern of SC errors and corrections.
Now, I feel alot better doing SC problems and feel that SC is truely the easiest part of the verbal section of the GMAT for the following reasons:
1- SC, as many have said, tests a limited number of grammer and english language rules and concepts. About 99% of these concepts are covered in the materials I mentioned earlier.
2- SC takes the least amount of time to master. For RC, for example, a long time should be spent reading Journals and well-written articles to speed up your reading and learn vocabularies.
3- SC problems can be solved the fastest compared to RC and CR, the remaining verbal questions types. Thus, mastering SC and being able to do SC problems fast, by quickly detecting the errors and finding the correct answer, can save you alot of time that should be spent carefully solving RC questions, for example.
Final Notes:
--> Make sure you read the official explanations in the OG's very carefully; and by that I mean for every correct and wrong answer choices.
--> Time yourself as your hit rate goes higher and higher for SC problems.
--> You are doing fine, your verbal is already 34

I took the GMAT 6 months ago and got 600 [Q 48, V 25]. Since then, I worked only on a limited number of difficult math problems and mostly verbal. I feel a huge difference in my SC and RC skills. Sometimes, when I do sets of 10 SC and 2 RC passages sets I get all answers correct !