A USC or LPR spouse may, not a girlfriend.

Hi,received 10 year green card last year based on marriage. My husband is green card holder. Am I eligible for citizenship?

No you would be eligible under the 5 year rule.

I751 pending ?

It can take between 18-24 months for adjudication.

Contact USCIS directly to inquire about your case.

The passport from when you originally entered the US should be used.

This can be corrected at the naturalization interview.

Which form should I Fill for Affidavit of support?

You should submit Form I864, regardless of whether you have sufficient income.

For form specific inquiries, it is best to have an attorney review both cases.

No this would not detrimentally impact your naturalization application.

Estimated case completion time april 2021 for n 400?

You may travel without issue assuming length of travel is minimal. Worst case is you can re-schedule your interview.