when a diode is forwards biased can current flow backwards through it?
this question comes from me trying to understand class AB amplifier biasing, so any additional comments about the diodes' use here would be appreciated
If a diode is forward biased, by definition the potential across it is such that current is flowing in the forward direction. In this situation there's no current flowing in the reverse direction.
If you suddenly change the potential across the diode such that it's reverse biased, and it's NOT a Schottky rectifier, then some reverse current will flow during the reverse recovery period. The current stops when enough charge (Qrr) has been delivered.
There's also a junction capacitance in all diodes which can allow signals with high frequency components to couple across the junction. But with a DC forward bias, current flows in the forward direction only.
In relation to your diagram, As Vin moves higher, the forward biased diode maintains roughly the same forward voltage and the drop across the top resistor is smaller. Actual current is always flowing in the same direction in the diode, but for small-signal AC analysis the current can be considered to be bi-directional around a DC operating point.