Dear Sir,
My answers are as follows:
1.what steps should I take to ensure that this will cannot be cancelled ?
Ans: Whatever steps you may take, the other party may challenge it and you have to produce concrete evidence before the Court and establish genuinety of Will.
2.What happens to this will in case if I end up switching to another religion from my current religion ?
Ans: It depends upon the terms of the Will, normally the benefits under the Will may not come to you if you change your religion.
3.What happens to this will in case if I end up marrying a girl of opposite religion ?
Ans: You can get benefits under the Will even if you marry a girl of another religion.
4.Can a muslim boy marry a hindu girl after mutual consent ? will both be considered hindus later for any legal matters ?
Ans: Yes, provided yourself converted into Hindu by approaching Arya Samaj.
5.Which religion will the children be considered to such a couple for legal matters ?
Ans: They have religion of father to which he belongs otherwise consent given by you that the children may adopt the religion of mother.
6.Is it ok after marriage if husband , wife and children remain atheist
Ans: It is also okay, except for filling few columns of school admission forms since there is no such column which recognizes Atheist as a religion. You may go to High Court in this regard and enlarge secular way of thinking. Whether you are bluffing all the above or in reality you would like to practice, I have a doubt.
7.Can such a couple travel outside india and have their government documents display this intercast religion ?
Ans: Yes.
8.I have to change my first name due to numerology, I know how to change the same in PAN , aadhar and passport, but changing the same in 10th,12th, degree is lengthy process , can next companies accept the gazette notification,affidavit for considering my education and experience certificates as genuine
Ans: After issuing a gazette notifications you may impress upon academic authorities to change your name in the respective academic certificates and if they do not oblige then you have approach a Court of Law.