For quant, focused, topic by topic practice tends to work best. For instance, you could do inequalities questions until you feel as if you will never again miss one, then do the same with sequences.
4 out of my 7 mistakes were in sentence correction, which is an area I do need to give time to. I am really not too excited about the idea of reading grammar rules and hence haven't really given time to it so far and am doing sentence correction mostly based on 'what sounds right', which clearly does not work on every question.
One thing that you can do to make reading about grammar rules more palatable is to look for patterns to the sentence correction questions that you did not get right. Not getting them right is annoying, right? So if you can find patterns to why you didn't get them right, you will be motivated to at least learn about the grammar rules that underlie those patterns.
In my personal quest for V51 I found that there were really only a couple dozen key things one has to know to get pretty much every sentence correction question right. I guess that something could have shown up that would have tripped me up, but nothing did, and part of the reason for that is that sentence correction is not a grammar test, at least not for the most part. Sentence correction is mostly a reasoning test that happens to employ not all that many grammar rules in the construction the questions.
Make sure you get parallelism. Make sure you get how changing the placement of modifiers can really affect what a sentence conveys. Learn the difference between restrictive and non restrictive modifiers. There are maybe ten things like that that you need to understand clearly and another ten or fifteen things that can help a lot too. After that the returns to learning new rules and concepts diminish.
Meanwhile, go back over the sentence correction questions you don't get and see whether you can figure out a way to have gotten each right without learning anything or anything much new. Most sentence correction questions are rather hackable, and by hacking I don't mean just going with what sounds right but also really analyzing the effectiveness of the wording and noticing what meanings are being conveyed. You might get some ideas on how to hack by going to my profile and looking through my sentence correction responses. I often outline how someone could hack a question without necessarily knowing many rules or idioms, and the rules and idioms I do use tend to be the ones that appear the most in SC questions.
Could you also give me thoughts on which other practice exams (apart from the GMAT Prep Software exams) are the most accurate in terms of the correlation of their scores with GMAT scores (or maybe a bit tougher than the GMAT)?
Try the tests made by Veritas and by Manhattan Prep. While their tests and questions are not always totally like the official ones, they are good for practice and the tests tend to for most people generate fairly accurate scores. If you find that some of the questions on those tests are not exactly like official questions, remember that reasoning and noticing details the most important skills for rocking the GMAT, and the questions created by Veritas and Manhattan do challenge you to reason and notice details.
