You may travel with a valid extension receipt while your I751 is pending.

Should I file for a fiancé visa (k-1)?

For an in depth discussion re: fiancé visas, best to schedule a consult with an experienced attorney.

What status should I apply for citizenship?

You would apply under the 5 year rule on page 1 of the N400 application.

Which one is better between motion to reopen n400 or reapply from the beginning?

Based on information provided, best to reapply from the beginning.

Best to have an attorney review your application prior to resubmitting.

Can I use my sponsor bank statement and what should he do so the USA consulate could believe me?

Please provide more details re; your situation.

I have B1/B2 visa and i would like to apply to get the social security number & amn't planning to work.?,

Unfortunately, you are not authorized to work on a visitor visa.

on form I-751 removal of conditions of permanent residence, is the petitioner myself the greencard hold?

The spouse with conditional residency is the applicant of the application.

Yes this is possible so always best to have an updated I864 with current tax year.

Yes you should have answered yes and should explain this at time of interview with documentation of final resolution of case.