Seromas can continue to produce drainage until the pocket collapses, so don't worry about that. I've had personal and professional experience with them. They are a nusuiance, but pretty benign.
Hope the Tegaderm helps. If not, try trimming the wafer so that there is enough of a dry margin away from the incision, even if it means a smaller area of coverage.
Jan 
P.S. Tegaderm is a cool product. I remember when it first came out. It was a really revolutionary product for dressings!