There was no water in the pan for my rib catastrophe, so all they got was a little sizzle on the aluminum wrap.
And yes, the bottom grate would have prevented the other rib from going into the abyss, but I took it out as it was one less thing to clean from the drippings.
IMO, I shouldn't have to worry about the top grate tilting in, and I've made sure of that now on my own.
The grate shouldn't fall of course. But, what I do if I'm cooking on the upper grate and not using the lower, I put a drip pan on the lower grate, and don't use the water pan at all.