Hi I am 19 year old I am soon to be marry I want to know if I could fix my mothers paper once I turn 21 even though am marry??

If you are a U.S. citizen, you can file for your mother when you turn 21, married or single.

Do we have to submit the I-864 with the I-865, or can we wait until the interview to show our financials/affidavit of support?

Yes, you have to submit the affidavit of support in your application for a family based adjustment of status. You can submit proof of your assets...

Do i need to file an ar-11 if i already got deferred action

Yes.

Is it safe to travel outside US with OPT status.

Yes.

Got marry after 3 months of meeting

In my experience, the officer will certainly take notice and ask you about it.

I had my green card from asylum since 2005 but I already went the my country 3 times. Now I want to know if I can apply n400

If your visits are evidence that you never had a well founded fear of persecution, then it is a problem. If not, it should not be a problem, but...

Can my family come visit me to the USA?

Yes, and you risk deportation when she comes as well.

It should be fine.

Would a CBP Officer be less inclined to grant a 3 month visit to the US if the traveler's visa (B2) will expire in 6 months?

It should not make any difference that her visa expires after six months.

Petition for reinstatement to F1 international student

It seems to be correct advice, there are plusses and minuses of either approach.