Hi
I just rewrote the GMAT and got a V45 (99%). That said, I had major issues with SC prior to this, I just could not get it right!! My english is pretty good, but my grammar is pretty horrid. Similar to Entwhistle above, I don't know any of those complicated terms mentioned in Manhattan.
What really helped me get increase my SC accuracy was two things, one of which you've already tried:
1) Powerscore SC bible - I read this first without knowing much about Manhattan at that time. I don't give this book enough credit, it did help me. I only read it once, but immediately felt more confident about identifying specific faults in sentences. My accuracy improved quite a bit post this. The books has section specific questions which, though easier, help you quickly practice the concept before moving on. The book by itself, is not sufficient as it does not cover some important topics. I would recommend one read though.
2) Manhattan - Initially, I had the same feeling, the book is tedious to go through and keeps throwing complex terminology at you. I never read the final advanced chapters, I felt bad the day before, but really, I'm guessing they're unnecessary after seeing my Verbal score. Acknowledging this draw back, I can not stress how helpful it is overall. I did something similar to your strategy - read a chapter, but then quickly jotted down a page or two of notes/key highlights (its boring grammar, doing this helped me stay focused as opposed to falling asleep in between), post which I would do random OG (or supplement) questions, not necessarily corresponding to the recommended ones. I know the question approach is not focused, but once you've completed about 5 chapters you've got a lot of the basics together. I also went through the exhaustive idioms list twice before going in, highlighting important ones (with a **).
I know I had two big problems in SC (simple/perfect tenses in present/future/past form and less common idioms), so if I encountered any such question, I would mark it. Even if I got it right, I would be sure to read the solution to see why the OTHER options were wrong too. I think that helped me as well. Perhaps try this?
Hope that helps!