Bipolar audio amplifier questions
1.) Does it make any sense to match the DC level between stages if the stages are AC coupled?


No. Each stage should be individually biased to get the maximum output voltage swing with minimum distortion. If you are coupling stages with electrolytic capacitors then a having a voltage difference helps to keep the capacitors biased (just make sure to install them with the correct polarity!).


  2.) What is the minimum limit for collector current? Is it just when the internal pn junctions stop being fully on?


In a linear amplifier the junctions should never be 'fully on'. The minimum Collector current limit is 0, but since an audio wave is AC you want the quiescent current to at least equal the peak positive and negative output currents. 

During positive peaks the output current is only limited by Base current x current gain, but during negative peaks it is limited by the pull-down resistor(s). If the signal is too strong then Collector current will drop to zero during negative peaks. This will flatten the bottom of the waveform and cause even-harmonic distortion. However you don't have to worry about crossover distortion, so the linearity at low volume should be excellent.

At best a class A amp is 50% efficient at maximum sine wave output, but with a resistor pull-down it is even worse. This is OK if you don't mind having a lot of heat for not much output power. For safety and reliability I would put heatsinks on the output transistors.