As the RUD event occurred during stage 1 flight the Dragon wouldn't having been travelling fast enough to need a heatsheild to survive reentry. (And from 44.5 km it hadn't really gone high enough to be said to be 'reentering', it never really left). The biggest danger to the dragon during the fall would be if it started tumbling.
The conic structure of Dragon is designed to take aerodynamic pressure from the nose on launch, and the base on descent, but a mix of forces in a tumble might have been too much for it. Whether it could have survived a tumbling separation and decent depends on how wide an engineering margin SpaceX used, and given how little we know it's difficult to speculate with any accuracy at this point.
As to your second question:
How much could have survived oceanic impact?
All of it. But it would be in lots of little pieces.
It's often said that when you're going at several hundred km/h hitting water is almost the same as hitting concrete, and it's true. Dragon is a beautiful spaceship, but there's no way she could remain intact after that kind of impact.
Of course it probably would have been possible (assuming minimal/no damage) for Dragon to open its parachutes as normal and survive completely intact. I doubt very much that this happened. A failure of this nature, with Falcon destroyed but Dragon intact, was probably never envisaged during the construction of Dragon's software.
The best we can realistically hope for is maybe some hull-panels, a Draco combustion chamber or two, and possibly a battered flight computer.