One option would be to set "set object physical motion " to false when
they release it, that way when they lower one is grabbed the physic
engine will not send the one on top flying. This will stop it from
falling down if the lower blocked is removed...
Not sure what the limits are, but i was having issues editing one of my
grouped objects. couldn't ungroup it or pull in another object into the
group. So I think there is some bug causing problems.