A little alarmist, I haven't noticed any reduction in wood quality. I cut my own wood, as well as buy from timber merchants. Every tree is not weak, or spiral grained. Osage is not all noodles. Some individual trees of any species fit into these categories, but they always have and always will.
Wood can crack when you try to bend it. It usually means you are doing something wrong, like using wood that has been kiln dried, or seasoned for a very long time. Trying for too tight a curve, without proper support straps. Leaving the wood too thick are all common reasons. Even some of the most experienced guys make these mistakes sometimes.
Tom you sound a little stressed, perhaps depressed? I'm not meaning to have a go at you, I have suffered from depression and have gone through periods where it feels like everything goes wrong no matter what.
I would have thought a carbon rich environment would be exactly what would help increase carbon storage, and density, in plants. I'm not a scientist, but the original assertion sounds counter intuitive.
All the trees mentioned sound like plantation trees. Conifers and oaks which seem to have less density than wild grown trees, many have a reputation for not being as good as wood from naturally regenerated forests. This reputation has been observed before anyone was ever concerned about man made climate change. Conditions that make good growing for a hardwood, can cause poor density for softwoods. The trees(hardwoods) with the highest density grow in warm tropical, and equatorial conditions, and are exposed to longer lengths, and more intense sunlight.
I'd want to see a lot more information before I start to go Chicken Little, and think the sky is falling in. Every couple of years the "experts" in the world of science tell us eggs are great for you. A couple of years later eggs are going to kill you. The same thing with fat or sugar, coffee, alcohol or whatever. What passes for science these days in mainstream media is usually sensationalist, doesn't show the whole picture, or is from a tiny sample, that it may not be indicative of the truth.