I would suggest you to wait. USCIS will issue an RFE if they want more documents. You can respond then.

I use FedEx. I think they are more reliable than USPS.

https://www.uscis.gov/uscis-lockbox-filing-locations-chart-for-certain-family-based-forms If you live in North Dakota, then you shuld send it...

If you have already filed I-130s with USCIS and it is currently pending (and if you have received receipt notices), wait until they are approved.

Cell phone repair shop sounds like more solid business but you might want to consult with an experienced immigration lawyer for details.

No restriction on when you can change your employer.

That message appears usually when your green card is approved. You might want to call USCIS and check if your green card is approved if case status...

You can be petitioned by an agent. There should be an agreement between you and the agent and a U.S. employer should recognize your agent to...

You should consult with an immigration attorney to find a non-immigrant visa that you qualify for as a Chef and submit COS before your tourist visa...

Please refer to this link: https://www.uscis.gov/i-131-addresses