A) Does a US citizen with a US passport require a visa to travel to Brazil for two weeks as a tourist?
B/C) Can a Brazilian citizen enter Brazil "as a foreigner" with their US passport only? If not, what needs to be done?
D/E) Can a Brazilian citizen without a valid Brazilian passport exit Brazil "as a foreigner" with their US passport only? If not, what needs to be done?
A) Does a US citizen with a US passport require a visa to travel to Brazil for two weeks as a tourist?
No. See TIMATIC info in Nicolas Formichella's answer and this visa table from the Brazilian government site.
B/C) Can a Brazilian citizen enter Brazil "as a foreigner" with their US passport only? If not, what needs to be done?
Yes. The Brazilian government site has this answered in their FAQ. If you show your other nationality passport and not your Brazilian ID ("RG"), you will be treated as a foreigner. This applies to both when entering or leaving Brazil.
D/E) Can a Brazilian citizen without a valid Brazilian passport exit Brazil "as a foreigner" with their US passport only? If not, what needs to be done?
Yes. See the linked FAQ above.
A) Does a US citizen with a US passport require a visa to travel to Brazil for two weeks as a tourist?
A) You could, but as of 2023-03 the Brazilian government decided to re-enforce visa reciprocity for some nationalities, US included, so very soon American travellers will need a visa to enter Brazil — as it was the status quo before 2019.
B/C) Can a Brazilian citizen enter Brazil "as a foreigner" with their US passport only? If not, what needs to be done?
B) Yes, but then you're treated as a foreigner and you'll need a visa. C) Nothing.
Note: you can enter as a Brazilian with a valid foreign travel document, if you also provide photo ID that proves that you're Brazilian (such as the Brazilian identity card). That said, it doesn't mean that you'll be able to board a plane with a foreign passport without a visa, as this combination might not satisfy the airline document checks. (It depends on what exactly will be the wording in Timatic once the visa requirements are in place.)
D/E) Can a Brazilian citizen without a valid Brazilian passport exit Brazil "as a foreigner" with their US passport only? If not, what needs to be done?
D) Yes. E) Nothing, apart from having to present any stamp or piece of paper that you were given on entry as a foreigner (used to control your stay, not sure if they're still using a separate paper for this).
Note: if the person in question is a minor departing the country, even if they bear a foreign travel document, and the Federal Police has reasons to believe that this minor is a Brazilian citizen (example: one of the parents is Brazilian or the place of birth is in Brazil), they are required by law to prevent the minor from leaving the country unless both parents are present or the accompanying adult has an special authorization given by both parents that allows the minor to leave the country (and this applies even if the minor lives abroad).
A :
TIMATIC states
Brazil - Destination Visa
Visa required. Visa Exemptions
Nationals of USA with a normal passport for a maximum stay of 90 days. Additional Information
Nationals of USA are allowed a total stay of 180 days in a period of 12 months. They can apply to extend their stay for an additional 90 days. Warning
Visitors not holding return/onward tickets could be refused entry.
B - C:
From this
To enter Brazil you must present a valid travel document (US passport) and a proof of Brazilian nationality (Brazilian passport even if expired, or any kind of document that proves Brazilian nationality)
As recommended in the comments, use your US passport as your main travel document for the airline and present your Brazilian passport for Polícia Federal as proof of nationality. No need to mention the expired Brazilian passport anywhere else.
D - E:
No, you will likely not be able to leave
From, this answer
However returning to the US will be a different matter. According to the Brazilian Consulate in Houston website :
Brazilian citizens, even those with dual citizenship, must enter and leave Brazil with a Brazilian passport.
