I'd start fresh with a new wheel rather than pay for all new spokes and labor. Sounds like the rim took a hit or two and is somewhat bent now. If so, it becomes a game of evenly tensioned spokes and a rim that is out-of-true vs. true only with some uneven tension (which is when spokes break). Your mechanic is saying that balancing act isn't working. If you want to try one more time - I'd suggest to the shop that you are more worried about even tension and not breaking spokes even if it means a wheel that is slightly out of true. Retensioning the spokes is a bit different than replacing one spoke and truing the wheel.