Can i complete medical exam now or do i need wait for Interview letter or rfe letter?

You should wait until the medical is requested to submit this, either at interview or through RFE.

You should be able to continue work while waiting for the extension. You can contact USCIS to obtain a I551 stamp if needed.

It is not advisable to enter the United States on a J1 visa with the intention to marry your US citizen boyfriend and later adjust status.

Yes, you may travel with your valid passport and green card.

Is there any way to adjust my status?

Yes, you may adjust your status despite an overstay. Best to consult with an attorney for guidance.

I m in a country where more than one marriage is legally allowed. My second wife has got theI-130 petition approved and waiting

Yes, it is possible to apply for your current wife however the previous marriage may be questioned. Best to work with an attorney for guidance.

The answer to the question would be yes based on the information provided.

Request a full copy of your file and consider hiring another attorney.

Yes you may submit a second complete and signed application if previous application did not have signature.

Is lt okay to not update birth certificate of a baby born to you after applying for citizenship?

You can update the information at time of interview without issue. Bring a copy and original of your child’s BC.