Contact a US immigration attorney prior to travel to review strategy and timing - you may have to do a civil ceremony state-side first and then...

Does an I-864 has to file for each individual?

Contact a US immigration for filing assistance - the number of I864s depends on the type of filing and your status.

At what stage of Green card process is the Birth Certificate required to be submitted

It is filed with the I485 (if in the US) or with the NVC (if IV processing). Contact a US immigration attorney for assistance.

Where to mail the I-131 Advanced Parole package ?

Review the I131 instructions at www.uscis.gov or contact a US immigration attorney.

Can i get a scholarship for my studies in the United States?

Contact the school and their DSO regarding scholarship eligibility.

Re application

There is no official rule - review the US Embassy's local website for local instructions.

Need advice

Contact a US immigration attorney regarding filing options and status review.

Contact a US immigration attorney for assistance since the child is not your biological child and IV processing will be necessary.

Can I apply for an OPT as my F1 status will be ending in May as I am graduating then and will it be issued.?

Contact your DSO and/or US immigration attorney for review & assistance.

Changing employer right after getting GC

Contact a US immigration attorney to review the general rules of "length of employment" needed after GC is issued.