Data wasn't going to be tested to destruction, anymore than a patient is treated to death. Data just didn't trust Maddox to take him apart and put him back together again without "killing" him. There's no evidence that Maddox was incompetent, just inexperienced. Data might not have been in any danger from him at all.
And I doubt that Soong developed his androids entirely independently. He would've had all the androids that Kirk encountered as examples of the engineering and programming needed. Maddox just wanted a live sample, something Soong probably lacked but overcame. Maddox could probably overcome that problem, too! Eventually.
Also, don't forget that Data didn't just show up at the Academy. He was found by Starfleet officers. And we all know that the rule is finder's keeper's. Therefore, Data belongs to Starfleet!
How do we know that he wouldn't have been tested to destruction? Data was obviously very skeptical hence his whole "I'm unique" speech. Yes Maddox could probably get there on his own but he was impatient and had no regard for Data's life to the point where he was willing to risk his life to get him there faster. A scientist that takes shortcuts is scary. I know that Data was found by Starfleet but he decided to join Starfleet as something of a tribute to them not because they owned him.