You say the second one "shattered." You mean you dropped it, yes?
Not knowing the whole story about these iPhones, but knowing lots of anecdotal evidence from clients, I have to wonder if some of these issues you're experiencing (which are, as you know, atypical) have to do with lighter, non-shattering drops, falls or being banged around inside a bag/knapsack/briefcase/et al.
Most of the people I've run into who experienced the oddball problems you're describing have either dropped them or let their small children play with the devices. I'm not saying this applies to you necessarily.
Extended warranty might be good ... might also want to consider an Otterbox case if you or the people who use the iPhone tend to be a little rough-handed with it.