I will be in a similar situation in Jan (fly to STT, stay one night, ferry or water taxi to Roadtown the next day).

Am I correct to understand that the 48 hour window for entering the BVI is just to avoid having to retest on arrival, and that otherwise you have a 5 day test window?

I gather that the USVI wants you to register your test results with their portal at least 24 hours before arrival there, which seems to push the edge of the 48 hour window for the BVI, even if it is really a 2 calendar day window.