Geof, that was one of the best summaries I have read on GW!
You are quite right that fully grown trees don't absorb CO2 and if they die and decay, CO2 is released. I might counter with "use the wood of mature trees to build houses for the continually increasing human population before the trees self-destruct. Then replant new trees." That is, if I really believed we need to do something to reduce CO2, which I don't.
My mantra of "plant more trees" is meant in the same vein as I might have said "go buy a Prius". If anyone believes this "we must act now" BS then let them do something that makes them feel good and is benign from my point of view. So, go plant a Prius.
The idea of artificial carbon sequestration scares me - that is if it holds CO2 for longer than the natural lifespan of a tree and its possible wood products. By burying sealed containers of CO2 we will eventually reduce the breathable oxygen available in air - remember the "O2" in "CO2" represents oxygen. Better to spew the CO2 into the air and let Nature find its own balance.