The uphill bonus (+1) is applied if one or more units are uphill of the enemy unit. In a multi-unit, 2 or 3 onto 1 combat only 1 of the units neeeds to be uphill to qualify for the +1 bonus.
Yes, only count the fron 2 stands in hand-to-hand combat. In the case of the popular billmen/longbow combined arms unit, once the two billmen have been lost the 2 lonbow stands fight as a 2 stand longbow unit. The longbow don't add their numbers but can be very useful in certain situations:
A good historical example of combined arms would be the battle of Crecy. Four stand units of longbow on the slope hex above combined arms unit in front on the flat behind a line of stakes. This give the English 6 longbow shots (4 overhead of the combined arms unit), for every enemy unit attacking and no impact bonus for the attacker because of the line of stakes. The 4 longbow stand in the rear unit can continue to shoot overhead as the front unit engages in hand-to-hand combat.
Another popular combined arms unit is crossbow, bow or handgunners behind warwagons.
Yes, when 2 or more units engage with 1 enemy unit you can use the highest applicable factors from both units. For example, a disrupted unit of mounted knights cannot count its stands in combat and has no impact bonus, but still uses its combat factor of 5, and if joined in the combat by a unit of peasants (combat factor 1), these extra 4 peasant stands are added to the hand-to-hand combat calculation.
The unit must flee in response to the combat outcome dictated by the combat results table. It does so at once moving a full move plus 1 extra hex. A unit which has completed the flee move will be disrupted. It can rally i.e remove its disruption marker and face any direction and adopt either a deployed or column formation at the end of its sides next movement phase after all other army movemnt has been completed.