I'm here with F1 visa, I want to apply for H1B in April 1st, if it got approved and I started work can I go home and come back?

Consult with your H1B attorney regarding specifics of your case. Generally it is not risky but it's always possible to run into visa issuance delays.

Although it is taxable, I think it's a stretch to call it employment. It's possible USCIS has addressed cash prizes specifically, would need to do...

Should be OK. If you can, request evidence from your employer of the extension petition being filed timely in case it comes up at the US embassy...

L1 Visa Maxout Reset

Yes.

What happens if a persons changes his - her citizenship from back home while on green card in the states?

No, unless to gain the new citizenship that person had to live outside the states and thus lost their US residency.

Is there a time limit to Visa Application through marriage license?

You can wait but the issue is her entering the U.S. as a visitor. If she plans to travel to the U.S. as a visitor, she may not be admitted. Talk...

My STEM opt extension will expire on 4th April 2015. Can I file H1-B on 1st April 2015 and have cap-gap & work during cap gap?

Probably, read cap gap USCIS memo...

Can we have two parallel/active visa application, filed by two different petitioner, at a US consulate at the same time?

The new application is going to also be on hold a until 221g issue is resolved. Talk to the employers attorney so they can assist you.

At US embassy visa has been denied section 221g of immigration law. what is green paper with below disrciption means. this mean

You have to wait. Probably there is a security issue that they are looking into.

Is it possible for a G5 visa holder to change his status to a G1 visa holder?

Yes, that's possible if the person qualifies for the new status.