Simple answers all in the CDC Order. Strongly suggest that anyone who may be seeking to enter the US carefully review the section for passengers.
1. No exemption for vaccinations. Very unlikely to see that until all passengers are vaccinated and community spread in the US is down below the magic 3%.
2. So long as the test you obtain prior to departure meets the three calendar day and other requirements for entry, you may use it to enter the US. WN will need to see it prior to permitting you to board at CUN.