How can my sister visit US?

One would need to know the reason for which "US embassy refused to give her permission to visit" in the USA.

Can someone with expired G1 visa that came to the USA as the son of a diplomat have a US citizen sibling petition?

Good question. Yest as to the petition. Not so much about adjustment. You might still seek immigrant visa since I am not sure you have accrued...

Can I send my citizenship photo copy now?

Wait now until the government either returns the file or requests the copy via request for additional evidence.

Can I work for my employer abroad while my H1B has been approved?

Rules are that your employment authorization abroad depends on the law of the country in which you stay. You H1B visa should not suffer under the...

My US visa is valid till 2025. My current stay will expired in mid of July 2019. I heard about going to Mexico and re-enter

You should seriously consider talking to an immigration lawyer about all that since your scenario does not seem very turned up for a favorable...

This will be an issue. You will have to explain why. If you have a legitimate explanation you should be fine. I assumed your question was about I...

How do I know when to Submit the i485?

Yes it means that your priority is current and you can file I 485. I assumed, of course, the abuser spouse was an LPR.

Im i j1 visa holder that is not subjected for 2 years rule can i go back home after getting married to a us- citizen?

I do not think it is a good scenario to follow. If you leave the USA, you should follow the consulate processing path.

USCIS does not really share this information with general public besides those estimated processing times you can find for some benefits on their...

Can I still file and send the forms (I 130, I 130A and I 485, etc) even if I filled them and signed them up a month ago?

Yes you can do that if for whatever reason you had to legitimately wait that long after the forms were signed to file them.