1) I recently got OG Verbal/Quantitative Review from a friend. Are the questions in these 2 different than OG 2015? If yes, whats the difficulty catoegory?
Yes, the questions will all be different than the ones in OG 2015 (or any of the big OGs). Those supplemental OGs just contain a different mix of real GMAT problems. The difficulty level and the mix of question topics should be roughly equivalent to that in the OG.
You don't need to use these, but they're a good source of extra practice, particularly if you're consistently getting in the high 600s or 700s on CATs and need more hard problems to practice with. In any of the OGs, there will be a mix of difficulty levels, so if you're a high scorer, the average difficulty of questions in the OG is much lower than what you'd see on the real test.
You should use the Mprep Navigator (which you may already have access to if you bought our suite of books) to search for problems by topic and by difficulty level, and to track your accuracy and timing:
https://www.manhattanprep.com/gmat/stor ... navigator/
2) To make the best use of time, should I spend last two weeks with QuestionBank of Verbal/Quantitative Review? Is QB worth investing? I can do either QB1 or V/Q Review.
There won't be much difference, really. They're all official problems. I'd recommend using V/Q Review guides rather than the GMATPrep question banks, though, because of Navigator. Mprep has more robust metrics, and answer explanations.
You shouldn't feel the need to "finish" any individual resource, though. Just a) do targeted problems in topics that need work, and b) do a series of random timed sets to build up stamina and pattern recognition.
3) Whats the best source of difficult questions which one can do in a week before test and that those questions are also like real GMAT Questions.
- use Navigator to find the harder questions in the OGs to practice with.
- if you're looking for a *serious* challenge on quant, check out the Mprep Advanced Quant Guide. Just note that many of these will be harder than all but the hardest problems on the real thing:
https://www.manhattanprep.com/gmat/stor ... mat-quant/
Good luck!