Hello, 

You can definitely score better in 12th and NEET as you have all your goals properly planned out. You should start your preparation from 11th standard itself. This will give you maximum and enough time for proper preparation for NEET as well as help you to get a good percentage in 12th exam. You can give NEET in the same year of your 12th exam and still score better in both the exams. It all depends on your confidence and preparation level. If you think you r well prepared for the NEET exam you can give it with your 12th exam or you can wait for 1 more year and then give NEET. This way you get extra 1 year for your NEET preparation. 

Hope this helps.

Good Luck.