1) In fact, this is misleading in the feature list. An overcurrent occurs when the load becomes short circuited or when the output is hard shorted (to GND, which is most probably). The chopper reacts immediately to an overcurrent, e.g. shorted coil condition by switching off, but it will retry in each cycle. The short to GND current can be influenced in a limited number of steps by tuning the short to GND detection time. This will influence the maximum current when switching into a short condition, only. If you need more finetuning, you can add a high side resistor to the highside MOSFETs (in series with each full bridge), but this will also increase power dissipation. It may make sense, when very small motors shall be driven and protected. A voltage drop of roughly 1.5V from VS to BM triggers the short detection.
2) There is no connection to STST, but OL will occur. It could be, that your power supply shuts down for a short moment due to the overcurrent, and this resets the driver.
3) You may load it with 20mA or even more, but output voltage decreases. Up to 50mA continously will not harm.