Is it mandatory that we 'file' income tax together?

You may very well need to file an I-751 waiver.

Can I Apply EB2 Green Card in OPT ?

Why would you need to file for OPT Stem extension if a visa is current? You can file an I-485 if you had F-1 status. You should consult further...

Waiting for EAD card to be re-send for 90 days! What to do please help!

Trying making an Infopass appointment early in the morning like 7:00 a.m. They usually have some time slots then.

Is it safe to write an invitation letter? Is it possible happen something to my status if any red flag shoes up?

There should not be any issues as long as you don't commit fraud.

Will the address not being changed be a problem?

In order to avoid any potential issues I would filed an amended return with the new address.

You need to get your employment authorization card before you can legally work. It takes about 90 days from date of filing to get it approved..

When do I apply for citizenship? Does the 3 yrs rule still apply to me even though I didn't get the 2 yrd greencard?

Yes - You can file if you have been married to an American for 3 years.

Visit the U.S. for Business purposes

Typically a detailed letter from employer stating the business-related reasons for the purpose of your visit to the USA should suffice.

Removal of conditions, how long the process

The I-751 cases can very well take more than 12 months to process. No need for concern. Make an Infopass appointment and get your passport stamped...

Am I able to re enter the United States with a felony?

We would need to review the criminal matter in greater detail. Having said that, you can try but at the airport they may find you inadmissible. ...