How is this different than the simpler example:
A: Readies to shift 5' away if attacked.
B: Attacks A
A: Shifts 5' per the readied action.
Then, is B's action cancelled, but not lost, or entirely lost?
Its ambiguous. If you are talking about B charging A then we don't know. Again, Charge is an ACTION and no rule covers 'targets' of actions. You could interpret it either way, that the Charge has a target and you can only attack that target, or that the MBA can be invoked whenever a legal target exists for it (after 2 squares of movement). If we're NOT talking about a charge, then this situation is trivial, B loses its attack. B has already ended its Move action and it no longer has a valid target for its 'Use A Power' Standard Action. I would note however that there are quirks here. A trigger can only be invoked on a readied action based on an ACTION of an enemy or a square of enemy movement. Choosing a target is a STEP of an attack action, thus A must decide if he's moving away before he knows B's target. Its arguable he doesn't even know what type of attack B is making when he decides to step away. Its even arguable that B can decide that AFTER A steps away. Thus the 'step away' tactic may be quite a bit more limited than some people might think at first glance.
If this is modified by:
A: Readies to shift 5' away if attacked.
B: Moves 20' (out of 30') putting B next to A.
B: Attacks A.
A: Shifts 5' per the readied action.
Then, can B continue moving the extra 5' necessary, then perform the attack?
No, once B initiates the attack he's attacking, not moving. This is true whether or not we're talking about a charge. Again, the question is if A's trigger happens before or after B chooses a target and a power to use (if its a charge then these things may already be apparent, but not always).
In either case, could B change targets from A to A', if they happen to also be adjacent to B?
Without specific rules covering 'targets of actions' and assuming a charge we just don't know. There are no specific rules that cover this. It is up to the DM.
Note the slightly different example:
A: Readies to move away from any opponent who moves within 10'.
B: Moves towards A (to make an attack).
A: Moves away from B per the readied action.
B: Continues moving towards A, reaching them (or not) depending on the remaining movement. Or, swerves to A', who can still be reached in B's remaining movement.
Yes, B can continue to move in this case, he hasn't completed his move yet and isn't obligated to end it. The 'swerve to A' part' is up to the DM still.
Or (depending on whether the equivalent of tanglefoot bags exist in 4E, and whether a charge can be continued if movement is impeded):
A: Readies to throw a tanglefoot bag on any opponent who moves within 10'.
B: Charges A from a distance of 20'.
A: Throws the tanglefoot bag when B is 10' away, and successfully hits.
Then, B's movement is halved. In 3E, that would prevent the completion of the charge.
Would B complete their motion to A (assuming 30', which is enough to reach A, even impeded), then be given a normal attack against A? Or would B stop immediately when the charge could no longer be completed?
This is 'an attack which slows the opponent'. The rules here are that the slowed creature's movement rate drops to 2. It can then continue with any movement it might still be eligible for (IE if it moved 1 square already it could still move one more square). If it has a legal target at that point, it can execute an attack. This isn't really ambiguous in the context of 4e rules, though again we aren't sure how the 'target of an action' is defined if this is a charge.
In all of the examples, would penalties for charging apply even if the charge could not be completed? (My gut says definitely yes, but, that means the charge is not entirely cancelled -- the charge has still happened to some extent.)
Thx!
TomB
This isn't really spelled out in the rules either, but I would say 'yes', if you charge then you pay the costs of charging, which is your turn ends as soon as you finish the charge. Again, the rules don't cover 'failed charges', so we really can't say what RAW is here, but it seems sensible to me.