I believe you Rip. Just because a scientific study hasn't been published stating that X amount of lean body mass has been gained in Y amount of time doesn't mean it is impossible. It just means it hasn't been published in a peer-reviewed journal.
Also, I occasionaly have to defend skinfold analysis of body compositon. Many people think that the more sites you use, the more accurate it is. The accuracy depends on the skill of the technician and the type of equation you select. If you are testing 18 yr old female volleyball players, there is a specific equation for that population. Use it. It may only contain three sites. So what, it will be more accurate than using an equation designed to test 45 yr old male bodybuilders, even though that equation might contain seven sites. It looks like you used a general population male equation (Jackson-Pollock), which will be accurate for Zach.
Also, you are spot on about the value of hydrostatic weighing. First of all, it is not easy to get someone to hold still underwater with all their air (save for residual lung volume) blown out of their lungs. Also, there might be air bublbes in their hair, swimsuit, etc. What if they have a rod in their leg from a surgery? Plus, most places estimate residual lung volume from a simple spirometery test. Only if residual lung volume is actually tested (nitrogen rebreathing, etc) can you start to think that hydrostatic weighing is more accurate than a trained technician performing skinfold analysis. And even at that, you would have to have all the factors of the underwater weighing go perfectly. After all that, the difference in accuracy is minimal (1-1.5%). Plus, hydrostatic weighing is expensive and time-consuming. But, it impresses a lot of people.