Hi Timmo,
We fight front rank vs front rank round 1 as normal.
Second round the reinforcements test to join an ongoing melee, then fight as normal with all modifiers, then the first rank fights minus charge bonus but plus any winning previous round bonus etc.
Third round if ongoing, rank three fights,then 2 and then 1.
Because the factors are all based on the unit charging/fighting it doesn't matter who they fight against as the factors are the same and casualties are spread as below on the whole.
Casualties are distributed each round in the order, 1st rank, 1st rank, 2nd rank, 3rd rank, 1st rank, 2nd rank etc.
We don't count units as disordered until the melee is over, not strictly correct but it keeps factors high enough to actually cause casualties and get a result.
Melee reaction counts the class of the "unit" as the highest proportion of surviving figures.
I quite like the little mini tests and 90 % of the time we don't even need to look at the table, very straightforward for charges etc as your not doing a full morale test.
I like and use card activation in other rules but with the system already having an order system I don't mix the two.
For Brigade Morale check out General de Brigade, they do it very well.
PM me your email address and I'll send you a copy of the play sheet.
Regards Ken