1) Based on the inputs on various forums I plan to concentrate on OG + Manhattan.
Which of the following two options is better:
a) OG(all books) + Manhattan(all books)
b) OG + Manhattan(all books)
OG12/13 have a fairly large number of practice questions, pretty much spread all over the place as far as difficulty is concerned (majority are easy to medium difficulty). The OG QR book and the VR book really do not provide a DIFFERENT spread of questions (More difficult ones for example) so it is MORE OF THE SAME. If after the OG12/13 you feel you may need more official practice (say you find quite a number of OG questions tough) then buy those dedicated books, however i would recommend starting off with the OG 12/13 and the MGMAT/Veritas Prep (include practice questions) set of strategy books.
Personally i feel there are better practice (Non official) sources available then the OG quant book, Veritas prep has a free Question bank for example, This forum along with others has hundreds of free questions to try out, and MGMAT Sets per topic can be bought for around 5$ a set, and include tough questions (majority 700+ questions)so having covered the OG quant book recently i would not recommend it unless one has exhausted quite a bit of other material.
I plan to take mba.com's diagnostic test on 15th Dec'2012. Whether I should prepare/study for the diagnostic test or should it be impropmtu ?
Spend 4-5 days refreshing your high school math and grammar, learn the basics of the test and what all is tested (topics and formulas in quant, logic, and grammar rules)..No need to go in-depth into each concept/topic, but do form a general picture. For example, do learn how to answer DS questions correctly (Memorize the answer choices etc)...
Is the 2nd diagnostic test required ?
If you study for 4-5 days and take a practice test on the 15th of December, there is no need to retake it again as that would only waste a test. Take the second test once you have finished the entire Coursework at least once (ie. covered all the strategy guides plus the practice questions ).
"When you want to succeed as bad as you want to breathe, then you'll be successful." - Eric Thomas