When considering the abductor theory the only reason for the assumption that an accomplice was involved is as a way of explaining the open window
We know, following forensic evidence, that no-one either climbed through, or out of that window ... so why was it open ?
The most logical explanation is that the abductor ( who had entered the apartment either through the unlocked patio doors or front door via a duplicate key ) opened the window shutters, scooped up the sleeping child, and passed her through the open window to the accomplice who waiting outside
Other than to accommodate that theory, I see no reason at all to assume there was an accomplice involved
Of course, this theory ( the passing of the child through the window ) opens the debate to further questions
If the child was passed through the window to an accomplice, why didn't the abductor simply climb through the window himself directly after ... thereby leaving with his accomplice to a vehicle parked in readiness ?
The use of a vehicle becomes almost a certainty if two abductors were involved ( one to restrain/hold the child whilst the other drives quickly away )
If Jane Tanner was mistaken, that it still a possibility ( apart from the abductor climbing out through the window )
If she is correct, however, and witnessed the abductor, alone and on foot, then the 'accomplice' theory makes little sense ( and the open window remains a mystery )