1. So the units are actually modular, i.e., not connected. Aha. If a unit jams, it's annoying but not fatal, as all the units above it continue to be usable.
2. Since the weight units are separate, then the force on the gearing (both wall and unit) is constant and thus the wear and tear is manageable.
3. The max energy stored is when all the weights are horizontal, of course. So there needs to be a transmission line built into the shaft, along with a motor/generator per weight unit. Some cleverness needed so the transmitted power is passed to (and received from) the correct units. The sliding contact where power is transmitted would also be a major possible point of failure.
4. Yeah, that's the downside of separate modular units. Each unit now needs to transmit/receive power.
5. It would seem prudent to build two or more shafts adjacent to each other. So if there is a problem in one shaft, you can go down the other and fix it. Increases reliability significantly.
6. If the units and power transmission are waterproof, water in the shaft isn't too much of an issue. You might lose ~20% of energy storage due to the buoyancy of the water, but that's it. Might save energy not having to constantly pump out the shaft.
7. Right. Definitely need a failsafe mechanism so a unit doesn't plunge down (say, when the motor/generator clutch fails) and take out the units at the bottom.
Cool idea. Good luck with power transmission.