Getting bumped is indeed a bummer... I know how it is when you have your heart set on a place.
On the bright side - Le Tahaa is a far, far nicer resort than the BB Pearl. And, with two days during the cruise on BB, there should be time to do most everything you were planning - you can rent a car and see the island in a half day, and if you go out to the motu, you'll be right next to the Pearl!
As for Bloody Mary's - you can either go for lunch when you rent a car, or there is usually a group that goes for dinner from the ship. It's a fun place but I don't think I'd give it up for dinner on the ship though - the seafood is very fresh but seriously expensive.
One more thing to consider - while a beautiful island, Bora Bora is a little cursed by it's popularity. While lovely people, there are many of the BB islanders who are a bit jaded by all of the tourists... while I find BB to be very beautiful, the people of Raiatea, Huahine, and Tahaa are so friendly that the overall experience for me on those islands really surpassed BB as a whole.
As you say, though, when something is your dream it's a letdown to have it disrupted. I hope Radisson can come through, and if not, I hope you will enjoy Tahaa.
--Rebecca