Not enough reforesting goes on, because of politics and cattle. 90% of the land that koa comes from is either owned by The State of Hawaii, or Bishop Estate. The remaining 10% is divided among smaller private land owners and most of them make money by ranching cattle. A good friend of mine has been ranching and milling for longer than I've been alive and he is a strong proponent of reforestation. At today's market prices, an acre of land will yeild more money than cattle in the long run, but you will have to wait at least 30-40 years for the trees to mature. Almost everyone who owns enough land to sustain koa is more interested in making some money now. I can't really blame them, since 40 years is a long time to wait.
Then, there's the political bs that Hawaii is not so famous for. We have had a few projects that were supposed to be pilots for sustainability, but none of them really made it to success. Most of this due to people not getting along, with their individual agendas.
As for the stability issue, one major benefit of a one piece top or back is one less point of failure. Even a good glue joint will age. Not having a joint in the middle eliminates one joint, in a major area. However, most book matches are more consistent across the width than an equivalent one piece. That's another reason why one piece tops and backs are so rare with the larger sizes of ukulele. It's already hard to find wide boards. Finding one with uniform grain, especially in a wood like koa is hard to find.
I've found that book matching does affect the tone, but it's not a night and day difference to me. The density and consistency of the grain itself has a much bigger bearing on the sound. We started doing exclusively one piece instruments in 1995, then started integrating book matches in 2005. Roughly ten years of one piece. At first, it was a 75/25% split, single piece to book match. Then, it came closer to 50/50. Now we primarily do book matches, unless the occasional wide board makes it's way over.