It is the Last leg that leaves for Bonaire. Here now. 100% sure that United travel ready specificity stated my PCR test was after 12:05 on Thursday or 12:05 Antigen on Friday (departure time for Bonaire leg).@drrich2 Bonaire consistently says the departure that matters is the one where you start the trip. Most people to be safe -- and demonstrably some airline gate agents -- use the final leg of the journey, i.e. the flight that actually goes to Bonaire, and try and get a test 48h (NAAT) or 24h (antigen) before that final flight. Depending on how you worded your query to them, it will be interesting to see their response. The bad news is that it may not matter to an ill-informed gate agent, who holds all the cards.
That segment you quoted from BonaireCrisis.com is NOT about layovers in the same country you started from; it is about layovers on an intermediate country, and whose rules apply.
The 48h or 24h is actual hours, not 2 or 1 days, and without fiddling with what clocks/time-zones say.
Your 0630 out of Nashville and 1030 out of MIA is no issue. You get your test within 48h or 24h before 0630, since presumably your test is in the same time zone as your initial departure airport.
ADDED: Most likely your testing paperwork will only be looked at by your originating airport gate agent, because they are the ones sending you to Bonaire and checking your luggage through. The gate agent in MIA will assume the gate agent that sent you to MIA did their job. Most likely.
They looked at date and time very carefully at landing in Bonaire but don't know what they were checking.
United wil probably not accept it via automated system.