1. Breakdown in oxide typically means crush of device, while breakdown of diffusion could be recoverable, depending on current density. So oxide breakdown is more sensitive.
2. It isn't clear what did you mean? If you mean difference in protection efforts regarding NMOS and PMOS devices, than is is explained by fact that NMOS have snapback behaviour, taking about all disharge current through themself and going to thermal fault first.
3. Mechanism of current discharge in that case is defined by design of protection structure.Typically parasitic NPN bjt will conduct most of current. NMOS turning-on will provide earliest switching of NPN.