A couple of things. I believe ferry service will be operating by July. That’s solves the airfare part. If it’s not available Dolphin water taxi has a hybrid plan where they will put you on a plane to EIS from STT to get to the BVI and then pick you up by boat at the Moorings dock to take you to STT. That’s your backup if the ferries don’t run.
As far as the quarantine part I also think that will not be required in July. The BVI has stated it intends to end the quarantine when they can get 70% of the population vaccinated. Unless they totally botch the vaccination process that should be done in May or sooner. If not your backup is the quarantine.
G
P.S. Negotiate with the Moorings on the extra days. Don’t pay rack rate. They probably will be running specials for extra days. I would try and get 2 for 1 at least on the 4 extra days.