I dont think by adding under bench storage will effect things too much.
I know my Aspect has under bench storage. If you have any doubt, take out the heavy item when moving it. I think they (eXisting Slide Motors you have installed) are designed to easily move the additional weight as long as within reason.
The Slides Roll on top of 2 each nylon rollers near each corner of the slide so the tiny motors that drive the assembly moving back and forth are doing so almost effortlessly.
Actually that Rack and Pinion design has some powerful Crunch when it closes.
as I recycled a broom head once in the back slide. ;D
I now take the closing and opening alot more serious and I pay attention!
As for your screw pulling out..Are you sure it wasn't OEM factory half assed work?
John