Welcome to Just Answers! Thank you for giving me the opportunity to assist you! I will do my best to help!
As a US citizen you are required to pay tax on your worldwide income. Since you are not earning anything in Mexico, you have no filing requirement. Your immigration status in Mexico is a different story. You would need to talk to an immigration lawyer in Mexico. I don't think that you are at any more risk than possibly being deported. Your uncle, on the other hand, is breaking the law when he gives the Mexican authorities the documents stating that you are working.
For your second question, your family can gift you a property. In order to properly address the tax situation, I need to know where this property is located, and are the current owners US citizens/resident aliens or not? From your point of view, in the US gifts are never taxable. When you sell it, the difference between what your relatives paid for it and what you sell it for is long term capital gains, and if you have no income, as you state, it would be taxed at 0% or 15%, depending on the amount of the gain.
For your third question, if you purchase inventory from the US, and sell it in Mexico, you are considered to be self-employed. You would report your sales, less the cost of items that you sold, less expenses that you incur, and pay income tax and self-employment tax on those earnings. If you are living in Mexico, you will be required to file and pay taxes in Mexico as well as the United States. If you live in Mexico and meet certain requirements under Internal Revenue Code §911 you can exclude up to $94k of income from taxation in the United States. And, you can use the tax you pay in Mexico to offset some of the US income tax (but not Self-Employment tax) that you might have.
I look forward to your reply so I can answer the second question! If you have any more, please ask as well!
Have a great week!