How can I correct my "highest level of education" on completed and submitted DS-260 form?

I would correct this at the time of the interview

How much does changing an interview location delay the N600 application?

Once you submit a change of address, your case if moved to the appropriate Immigration Office. the scheduling of the interview will most likely be...

How long is take to approve my I30 and I485?

I would start with submitting a case inquiry: https://egov.uscis.gov/e-request/Intro.do and then consulting with an immigration attorney. There are...

Our firm has been able in the past to obtain B-2 status based on medical reasons.

Out of status for a year applied for asylum based on conflict and torture what do I need to prepare for my master hearing?

The best advise I can give you at this moment is to obtain legal representation from an immigration attorney experienced in handling asylum cases...

Can I get a green card through family petition if I have a U visa?

A lot more details are necessary to accurately answer all these questions (like, how you entered US, etc.). You would highly benefit from...

Is there any chance for me to keep my green card after not entering the United States with it in 8 years?

By moving to another country, intending to live there permanently, you lost your permanent resident status.

Can I apply for a new EAD before my OPT expires?

being engaged to a US citizen DOES NOT allow one to file for work authorization

What do you suggest I do?

Your situation is not unique, there are a lot of similar cases. I would suggest you discuss your situation in detail with an immigration attorney...

How can I continue to work in the States after my OPT expires?

K visa/status is not for your situation. However, you may apply for adjustment of status once you marry your US citizen fiancee. You should consult...