Can we travel before Citizenship?

No you cannot file for U.S. citizenship at the U.S. Embassy in Spain.

I am on F1 visa and my boyfriend is GC holder, we want to get married without applying for immigration, can i re-enter us then?

It could affect getting a non-immigrant visa, if it appears that you do intend to immigrate to the U.S.

Can my fiance stay in the United States while his passport renewal processes?

No, applying for a new passport is not a basis to stay in the U.S. Marriage to a US citizen and filing for adjustment are a basis to stay in the U.S.

Reaching the end of my 180 Days, how would I file for an extension? Unexpected situation.

If you got an I-94 card [even one you could download from CBP], you can file an application for extension of status. Otherwise, the only way to...

If he is still and status and the visa becomes available, he can file for adjustment.

How long do I need to stay in the US to preserver my green card?

There is no black and white answer. If you leave for 3-5 years, you certainly risk abandoning your status.

How to change the petition after filing immigration form I- 130.

Yes, your father can file for the other son.

My husband had applied I130 this year and has been approved and gone to NVC.

No.

Does withdrawing a i539 save me from COS Denial? Does a COS denial stop me using VWP and ESTA in the future?

If you filed the cos on time, you will accrue no unlawful presence if you simply leave. But if you withdraw it and then leave, you will accrue...

Aunt had citizen appt with immigration and was asked for documents she doesn't have.How she can do it in order to be approved.

The trouble is that her claim is not plausible. The officer is not required to believe that that U.S. is her primary residence when all evidence...