Can I leave the United States with an expired A2 visa without any problems, and what will happen if I try reentering the US?

If your application has been denied then you are out of status. You will be denied entry.

I am filling for H1B visa on April 1st '14 and planning to travel to my home country(India) from April 5th to April 29th '14.

As long as you have a valid visa you should be fine. However, consult an attorney if you need more case specific advice.

If my application for adult adoption gets approved, will I qualify for a greencard or citizenship...??

Ufortunately, your adoption as a 45 year old will not allow you to get a green card as it was not done before you turned 16.

Currently on OPT. Would you suggest that I contact USCIS to clear things that might become issues when applying for H1B lateron?

Your application will have to be filed first. Then you can clear things up as USCIS requests information is needed.

What happens to student visa of a student if her husband's h1b expires?

Your F visa is independent of your husband's status.

Can a derivative child file an I-485 seven months after the principal beneficiary receives her green card thru F2A petition

She can join in by filing a petition, but you should consult an attorney.

It is a good thing to be above the federal poverty level. What is the problem?

Petition for green card approved but ineligible to adjust status. What are my options?

You should take form I797 and take it to an immigration lawyer as soon as you are able. Good luck.

Do i need to change my Last name 1st before i can petition my husband?or do i need to change my marital status?

Changing your name is a matter of preference. You can apply for your husband as soon as you are ready.

Can I marry my girlfriend (US citizen) after entering the US on a tourist/ visa waiver program. Or do I need a financee visa?

Coming to the USA on a tourist visa while planning to stay would amount to visa fraud and could come to bite you. If you're coming to marry, you...