Here's my take on the whole Mantle's Approach thing. It had incredibly strong shields and good armor, but a very squishy interior. I heard somewhere that up to two thirds of the ship were hardlight, to allow for variable geometry, such as that displayed when the Didact very quickly reattached the composer, and then the various moving pieces during and after the trench run. So, it would be extremely hard to get past it's outer defenses, (I'm pretty sure the Infinity was only able to penetrate the armor around the composer because Chief disabled a localized shield generator over a section of relatively weak armor plating, considering that the entire section of ship was just recently added/reconfigured around the composer). So once the nuke detonated, it blew the surroundings to smithereens, but also probably disabled most of the systems ship wide, including most of the hardlight (or maybe that energy was what Cortana used to shield Chief from the blast?). The remaining parts of the ship just kind of fell apart