1) It depends on which Kaplan CAT you took. If it is a new test, they are actually very accurate. If it was an old test (pre Nov 2010, or on a CD) there is a chance that it will severely underestimate your score.
2) It is impossbile for someone to give you advice medically, but here is my personal advice. Don't take it, becuase it makes you nervous. The last thing you want is to be is nervous on test day. The only way I would use it is if you have tried it during your studying or a practice CAT and it helped with your concentration. I am a coffee drinker, and I found that if I drank coffee before a test it actually HURT my performance. I made sure to stay away from it during my studies and test day.
3) I used the Kaplan Strategy and it really helped. Even though I didn't always go back to the notes, it really helped me to put things into my own words quickly. I usually did one sentence of notes per paragraph, and always made sure to look for keywords. I would practice at work when I read articles online, and even think of possible questions I'd get on test day... I went from getting 3 right normally to only missing 1 or 2, and brought my verbal up dramatically
4) The GMAT does not test your ability to know grammar rules or names. They only test your ability to pick out gramatically and stylistically wrong sentences. I would recommend that you use Kaplan's method for SC, and learn about all the types of questions you could be asked. Learn what to look for, and also purchase the Manhattan SC book. This is the holy grail of SC. Also, don't worry too much about memorizing idioms b/c they are not tested nearly as often on the GMAT anymore. I also would hold my pen up to the computer vertically and move it from left to right accross the answer choices to help me pick out "splits".
5) I would use Kaplan's method, and also put a lot of emphasis on Kaplans Advanced CR methods once you get there. They really helped me, especially with my timing. I found myself writing down ATLEAST the question type (weaken, strengthen, etc..) and a couple quick notes. I thought about the conclusion and scope, etc... Sometimes knowing wrong answers is just as helpful as knowing right answers. Learn the common wrong answer types (out of scope, 180, extra info, etc..) Kaplan's method is good for these also
How do I know this? I took a Kaplan course just a couple months of ago (and read 3 of the
Manhattan books). I did almost all of the homework for Kaplan minus a CAT test or two. Went from a Diagnostic of 570, to a range of CAT tests between 600 and 690, and scored a 710 on test day. I also found the Quizbank and OG Matrix very useful for Kaplan, and went through almost all of the "Hard" math questions, and went through every question in the OG (minus some of the RC passages). I also did the test day experience at the pearson center where my real test was. Good luck! and come back if you need help
Also read my post here to really help step your mental game up

:https://gmatclub.com/forum/10-tips-to-improve-your-score-app-with-no-extra-studying-124393.html