Charge! Flee! Redirect! Catch. So many doubts!

First: You have to redirect into a viable target. So, if the hydra could only be charged by going through the dark riders, it is not a viable target, and the enemy unit could not redirect to it. Now, if based on location, facing and movement the enemy unit can go around or bypass the dark riders, it could redirect, but it would still have to go around the dark riders.

Second: you can not come into base contact with an enemy unit that you did not declare a charge against. Effectively, redirecting is changing your charge target.

Lastly: A unit that completes a charge against a fleeing enemy will stop where they touch. Regardless of any movement remaining. They may then attempt a Ld test to reform. The unit will move no further, whether they reform or not. A pursuit move (from a broken combat) follows different rules.



“REDIRECTING THE CHARGE
If the target unit declares a Flee! reaction, the charging unit now has a choice: it can either attempt to complete the charge against the now-fleeing unit, or try to redirect its charge by making a Leadership test. If the test is failed, the charging unit must attempt to complete the charge against its original target. If the test is passed, however, the charging unit can declare a fresh charge against another viable target (the chargee may declare a charge reaction as normal). If there are no other suitable targets to charge, a charging unit cannot test to redirect.
Each unit can only make one redirect per turn.”

“CHARGING A FLEEING ENEMY
If, for whatever reason, a unit completes a charge against a fleeing enemy, move the charging unit(s) into contact with the fleeing enemy as described earlier and then the fleeing unit is run down by the chargers. Most of the unit is hacked apart or trampled to death, and the few survivors head for the hills with no intention of fighting further – remove the entire unit as casualties.
A charging unit that destroys fleeing troops in this way will often find itself pulled out of position and vulnerable to enemy counter-charges. Accordingly, we allow the charging unit to attempt a reform by passing a Leadership test. If the test is failed, the troops are so overcome with hacking at the fallen corpses that they do not have the chance to reform. If the test is passed, the unit immediately makes a reform manoeuvre. In either case, the unit cannot move further during this Movement phase.
”

Thx for the clarification!
I have one last (for now!) question :

A charge redirection must be declared BEFORE or AFTER the fleeing unit has been moved on the table?
I think it's AFTER but the rulebook is not so clear in this regard, at least for me : )