I'm a green card holder. For how long I can stay outside of USA and still being able to go back to USA ?

Assuming that you don't have a reentry permit, you may spend < 6 months outside.

For a new office, the employer will have to start with L1A. Subsequently a EB1(c) may become a possibility.

Getting married to a US citizen

not necessarily, if you have solid evidence demonstrating a bonafide marriage and the 3 year relationship. Highly recommend working with an...

My name is Goutham. Currently i am on STEM OPT extension. Failed to submit the Employment Validation Report on time to DSO.

Contact your DSO asap. They can advise on the steps if any.

The O is granted to allow work in the area of expertise. Uber work will be considered violation of the terms of your visa.

H1B stamping for Indian national in Sofia, Bulgaria

Yes, it is possible. Your firm's immigration attorney can assist with the documents.

How can I proceed the RFE response without client letter ?

It may certainly be possible. Please have your company get the RFE reviewed by an immigration attorney to determine a path forward.

I filed I-485 for my mother and got RFE to prove relationship. What can I do when my birth certificate only has father's name?

A DNA test to prove paternity would suffice but there may be other options. I suggest that an experienced immigration attorney reviews your initial...