Cool item. If I may comment on the Rolemaster question on slam.
Just treat them allowing a Huge Bash attack with an OB set to your desire. Given the downsides on the item, a +30 OB may be fine.
The weight penalty would be like carrying 40 lbs of weight (20 lbs per glove).
As this is inorganic matter, this will cause problems for some casters :)
The crumble attack could be a medium bash using thrown spear range. Go to Comment
Wow, this shield has a rather dramatic side effect.
I'd suggest the following:
When the shield catches arrows in this manner, it stores the damage and slows dissipates it while the shield is held. If taken off before the energies have dissipated, then the bearer takes the damage. Rate of dissipation of course is left to the GM, say 1 hour per arrow.