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).