mistermaster

It's okay, you can send all the forms (I-130, G-325a for petitioner, I-485, I-131, I-765, I-693, G-325a, G-1145, et. al.) at once.

You also have the option of paying the processing fee by money order --- $420 for I-130 and ($985 + $85) for I-485. You can't get a money order that is more than $1000. Make sure the names are well written on the money orders. Signature required on all orders as well

Send a notarized copy of the birth certificate from the National Population Commission in your country. You can notarize at a bank or other designated places.

Label the copy by putting an handwritten label with a "birth certificate" text on it. You can use a POST IT NOTE for the label.

No need to send a cover letter. Just make copies of the RFE notification but send the original RFE notification with the notarized copy of the birth certificate.

Include photocopies of the birth certificate and passport page for the spouse. You can include copy of the visa page as well. If you have more time, notarize the copy of the birth certificate at the bank.

Yes, it is required if the petitioner or sponsor of the applicant's green card is a United States citizen. Check column 2 of page 2 in the instruction document. It is under the headline --- "What Documents Do You Need to Show That You Are a U.S. citizen"