No test for 25% losses are ever taken during the combat phase (including odd situations when casualties happen not directly because of combat).

In the Declare Charges part of the Movement phase, first declare all normal charges and charge reactions. Once this is done, measure the ranges to see if Frenzied units must declare a charge and to see if Fear and Terror-causing units are in range of their targets, as these psychology rules may affect the declared charges and charge reactions (see also next question). Once all the charges and charge reactions have been determined, proceed with the Movement phase's normal sequence.

Yes.

Directly away from the Terror-causing creature, as per a test failed at the start of the turn.

They must wait for all the non-frenzied units to have declared charges (including reactions).

As with characters (see "Mounts' psychology" on page 79), the entire model is affected by the psychology rule. This means that both riders and steeds (in case of cavalry), all crew and creatures (in case of chariots) or both the character and the Monster he's riding, gain one Attack from Frenzy, can re-roll their attacks for Hatred, etc. Note also that if a part of such a "multiple" model causes Fear or Terror, or is affected by Stupidity, the entire model does (for example, only the beasts pulling a chariot may be stupid, but if the model fails its test, taken on the crew's leadership, all of the model is affected by Stupidity, including the crew). Note that the Mark of Khorne in the Hordes of Chaos book is an exception to this, as it clearly states that it only confers the extra attack to the rider/crew and not to the mounts/creatures pulling the chariot.

If the screening unit is also declaring a charge (i.e. they intend to get out of the way), then the frenzied unit behind them can, and indeed must, declare a charge as well. If the screening unit does not declare a charge, the frenzied unit behind does not have to declare a charge, as it wouldn't be able to reach the intended target. If the screening unit were not to move far enough to clear the way (because they failed the charge, or if a Fear test stopped them in their tracks, and so on), then the frenzied unit behind them would also fail the charge, stopped by the screening unit getting in the way.