Just to add to Barry's answer for Question a) Page 21. LOSERS - Cavalry losing a melee test morale and are routed if they fail. So you may not even get to a subsequent round of melee.
Question b) Page 22. The "all or nothing" charge - Where a regiment of cavalry charges and has more than 1 squadron in base to base contact with the squadron behind it and upto 4 squadrons, all the front squadron fight, half the second squadron behind it and 2 figures from each subsequent squardron so 6 + 3 + 2 + 2 = 13 figures fight in melee.
All casualties received in the melee on the regiment are divided between the number of squadrons, so in your case (6 casualties divided by 4 squadrons = 1.5).The average number of figures per squadron now is 4.5 rounding up to 5.
If the regiment lost the melee then a morale test must be taken (Page 24)
and a score of 5 or less is required if the regiment is Elite, 4 or less on a D6 if not.Add or subtract modifiers where applicable.
To pass a morale test it must score less than the remaining number of figures in the squadron/regiment on 1 D6.
Barry I have a question.....(Page 24) Cavalry Morale - The first circumstance states.....It has taken casualties this turn.....Does this mean that in a melee even if the winner of the melee takes a casualty it must take a morale test.