Assume the undead achieve 4 wounds on the chariots.
Assume the Orcs achieve 2 wounds on Unit 'B'
Assume the Orcs achieve 15 wounds on Unit 'C'You can never do more unsaved Hits to a unit then it has in total. (3x3=9, normally)
So 4 hits, 2 hits and 9 hits.
Combat resolution is
Orcs achieve 17 wounds
Undead achieve 4 + 1 support
12 wounds in favour of the OrcsThe calculation-theory is correct, but the numbers should be 11-5=6. (given one stand supports correct)
If the shooting attack from the spell carries over into combat, you have to check if it also counts towards combat result and in that case it should be: 12-5=7.
Unit C is wiped out,
Unit B loses one stand 2 wounds in melee + one shooting.
Wolf chariots lose one stand and carry a wound.Also correct, if the magic-shooting attack is carried over into the following combat round, which also can be found in the spell description.
This is the first round of the melee, so the orcs are permitted a 20cm move if the enemy is wiped. However the enemy unit 'B' stil exists so what happens now?Boar Boys were not fighting unit B, so they are free to Fall Back (3D6) or Advance (=new charge move up to 20cm to the closest viable enemy).
In which direction should unit 'B' move its 12cm?A unit moves directly away from the units side where it has the most enemy stands in base-to-base.
If a draw in number of stands, the commander of the falling back unit decides.
So, in this case, it is touching only one unit, Wolf Chariots, and is doing so on a corner-to-corner, so it gets to decide.
The distance is not correct. It should move combat-result divided by how many units that fought on its side.
So 7/2=3.5, which rounds up to 4cm.
I assume if Unit B had to move towards Unit 'A' then 'A' would move out of the way, unless A was in combat and unable to move then in that case B would be destroyed.Yes. However A can choose not to move at all and then B gets destroyed.
However I believe Unit 'B' would move to its table edge.In this case, by choice, yes.
Must Chariots re engage unit B or may they choose to take on unit A and if so would it be a flank attack?
Can Boar Boys re engage unit B or choose to take on unit A?B were not wiped, so Chariots can choose to Fall Back or Pursue B.
Boar Boys can Fall Back or Advance-Charge the nearest of the two remaining Undead units.
In effect would it be possible for either:
the chariots to swing into unit 'A' whilst the Boar boys hit unit 'B',
or
indeed for the chariots to purse unit 'B' whilst the Boar boys move forwards to unit 'A'.
or must both orc units go for Unit 'B'Chariots must wipe B first, before doing any Advance.
Boar Boys can Advance against which one is closest, when it is their turn to move.
Remember that if you choose to move Chariots before Boys, the situation may not at all become the same as if you choose to move Boys before Chariots.
Line of sight may become blocked for advancing unit, enemys front may become blocked by the first unit that already moved, and so on.

Probaly some of our fellow Warmasters here can find some faults above, but hopefully the mainpart of the answers are correct.
