If I go through your mock test scores, clearly, your verbal scores on the GMATPrep tests
have been far lower than those on your MGMAT tests. I think that's telling. My personal belief is that the scoring algorithm for MGMAT (both quants and verbals) is more lenient and can lull you into a false sense of security.
I think, with hard work, you CAN notch up your verbal score significantly. A 650+ score should be a distinct possibility then, given there's a lot of room for improvement in your verbal score.
I think you should concentrate on doing your verbal prep differently, as your current method has not worked twice.
I think you can have a look at e-GMAT
. It's relatively cheap and is targeted at non-native speakers primarily, but there's a good chance you may be benefited as well. People who have taken the course seem to recommend it highly. You could possibly try their free sessions first and see whether you like them.
Please go through the Brutal SC, CR documents on GMATClub - and also the verbal notes by whiplash, Gayathri, spidey etc - all are free downloads. You can download them here: download.php
You could go through the GMATClub SC, CR and RC and PS, DS discussion threads, which are broken down into 700-level questions and 600-level questions. You can find these links here: viewforumtags.php
Also, please go to whiplash's profile and download all the verbal documents you'll find in her signature.
All the best!
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