Hello,
See if you have cleared your executive and enrolled for CS Professional, before expiry of your registration period. Then it does not invalidate your Dec attempt. As after CS Executive it usually take a period of 9 months to sit in professional exams. As you have not mention when you have clear your executive so it is tough to write here then you are eligible for Dec attempt or not.
2. As per your second queston, if you have clear your Executive after 25 August 2015, then you have to follow NEW MODIFIED trainings. For details you may contact to your nearest chapter of ICSI or you may contact ICSI Helpline at 011-33132333
3. As your last question, SALARY does not depend only on the degree of a person, it consist all of the details like Professional qualifiactions, skills, experience, atttitude and many more. It is hard to predict what you can earn, but i can say that you will earn good enough.
Hope i am succeed to deliver the answer for your queries.