I don't know much about deep sand beds. Mine is the recommended 1.25 inches. But you can modify the package any way you want. I didn't use TBS sand because I wanted black sand.
Refers to something going south in your tank causing your water to way out of whack and things dieing off or having a close call. For instance those sea apples we're discussing in another thread. If they put out their toxin they could cause a tank crash. A temperature spike could cause a crash, spilling coppersafe into your tank a big crash...etc. etc..
I usually change out a dsb about every two years or so. I'd also say it really depends on how many critters you have in your tanks. By crash they mean start to release large amounts of nitrates into your water colum causing a cascading effect "everything goes to that fish tank in the sky." I would also say that I had a reef that I didn't touch for 5 years that had a dsb but, it also had no fish so.......