MF2 moves are permitted in the movement phase SP4.3 (which will be after any charge combat) or in the fighting phase SP5.5 (after melee combat). Check the turn sequence for more details.
They are permitted to any UG that has been contacted from a flank or rear charge and is not in frontal combat.
Your own charge is cancelled as soon as you are contacted by the enemy charge.
So in your example, the enemy cavalry has contacted you in the flank in SP 2.6. The charge combat will be fought (SP 2.7) and your opponent will have a +4 claim for a flank charge.
In the movement phase (SP 4.3), you can turn to face them. If you don't then the cavalry will still have a +2 claim for the flank, however having turned they do not get this claim and you may have a claim for a weapon.
So to summarise:
a) You cannot face the charger until the movement phase.
b) If you are already fighting an opponent to your front, you cannot use an MF2 move.
c) No you cannot charge your original target, if you are contacted by a charge. The order of charges matters in this situation.
Hope that helps.
(For anyone who plays Maximus or Magna, it is worth noting that the rules in Pacto are different. In Maximus and Magna individual bases can turn, in Pacto the whole UG turns.)
Richard
