I consulted a knowledgeable tax accountant based on a recommendation of o friend of mine who is an accountant. If you have a dependent work relationship with a U.S. company that does business in the U.S. and not here (including if you are a contractor), you do NOT have to pay Argentine taxes at all, even if you are a resident here. So no worries there.
Your other information is absolutely correct. You are exempt from income tax up to 90,000 if you are outside of the U.S. for 330 plus days of the year. You can claim the foreign income exemption on your tax return. However, note that you will still have to pay social security. You may also claim this exemption if you are a bona-fide resident. If you have permanent residency here and can prove it to the IRS, then you can also claim the exemption, even if you are here for less than 330 days.
This means that if you are a contractor or have a U.S. based corporation, you do not have to pay income tax in Argentina or the U.S. if you make less than 90,000 USD per year.
From what I understand, you can do a regular IRA, but not Roth IRA. But since you won't have to pay income tax, you shouldn't need the Roth IRA. I'm not 100 percent certain on that, so you might need to ask an international accountant.
If you are working in an Argentine company here, you will have to pay Argentine taxes either through a monotributo or ganancias.