Audio power amp oscillogram gap
This is normal behavior that closely models real life behavior. What you see here is a result of overloading and forcing some transistors from active region into saturation. In this particular amplifier topology here what happens under positive overload:
 1. Q1 goes off, Q12 gets saturated, because there is no more current flowing in it from Q1
 2. Q6 is off, Q10 is off, Q11 is off
 3. Current source Q4, Q5 is not loaded any more, Q5 saturates.  

It takes some time to bring Q2 and Q5 from saturation to active region.

C3 is there to stop oscillations. Simulation lagging typically indicates oscillation, but it has to do with your software and transistor models. Try increasing it or playing with simulation parameters.

R5 being 10k is a good guesstimate. It is there to bias Q4.

Two diodes in top output stage, four in the bottom.