I've never seen one of these things rolled, can't really comment on how the top holds up. I would assume being fiberglass it will tend to splinter, and that would probably only happen if the rear support in the top failed. It probably would IMO, since they seem to rust pretty good, what little you can see of it.
Based on the roof design and bed length, were it to go upside down, particularly for an infant, the angle between tailgate and roof wouldn't allow impact (on relatively flat ground) for the rear seat, even were the top to completely fly off.
Driving habits are going to be far more of a concern than the potential repercussions from rolling it IMO. Not saying you drive bad, but having to react last minute is OFTEN a result of poor driving habits. Not always of course, others do stupid things, but tailgating, speeding, etc., are things we can control. No matter what we are driving, we can't control what others do.
I had my Daughter out twice with the top off, and many times with the top on. Safety never really factors into my decision to drive the truck. Crumple zones are great, but if I and my Daughter were cloned, one pair in the econobox, and one in the K5, and there was a head on collision between the two, I know which occupants I'd wager on surviving with the least injury. Things start to get messy when the mass of each vehicle is more evenly matched.