The first set of diodes protect the MOSFET gates against overvoltage if the output gets shorted. The gate insulation in mosfets is very thin. It takes something like 50V to blow it out instantaneously, but the long-term odds of failure start to rise as soon as you get over about 12V.
The second set, as far as I know, are some sort of clamp to give cleaner clipping behaviour when the power amp is overdriven. They stop the driver stages from saturating, so they recover quickly from overdrive without hanging up. That means the user can overdrive it a fair bit without even noticing, giving the illusion of more headroom.