No, you just need an active passport to travel to Brazil. No visa is required.
YES. While Brazil’s COVID-19 entry regulations no longer require proof of vaccination, the CDC recommends that U.S. citizens not travel internationally until they are fully vaccinated. See up to date information here: https://br.usembassy.gov/covid-19-information/
YES, but only for those who are unvaccinated. See link for details: https://br.usembassy.gov/covid-19-information/
YES
None required, but see Health section here: https://travel.state.gov/content/travel/en/international-travel/International-Travel-Country-Information-Pages/Brazil.html