I have used Alloderm for a variety of reconstructions and it does incorporate. In instances of inadequate fluid drainage or infection it will take longer to incorporate of be unable to do so until the wound has been treated appropriately.
Yes the part that is not attached can be free-floating. However it is pretty rare to have the entire piece not incorporate. Therefore, even with only a few spots incorporate they will hold the entire piece of Alloderm in place. If it was free floating you would be able to feel that as a mobile lump and I doubt that will ever happen.Good luck!