First, remember that when you are responding to "after a challenge is declared/initiated," that has to be played in the response step for the "initiate challenge/declare attackers" framework window. Any redirect that comes AFTER that point is going to come too late. The redirect itself is not considered to be initiating a challenge, so it doesn't create an "after a challenge is declared/initiated" response opportunity. That means Lord Commander and the Supports mechanic come too late to affect the kind of response you are talking about, period.
Example: If A attacks B and C redirects the challenge to himself with the Lord Commander title or the Supports mechanic (both would happen as part of declaring defending characters), C cannot trigger his Euron or Bringers of Law because it is too late to trigger "after an opponent initiates a challenge against you" responses.
Next, remember that the play restrictions for responses are checked when the response is triggered, not when the response opportunity is created. So "after a challenge is declared/initiated against you" responses only care who the defending player is at the time they are triggered, not who was named as the defending player when the "declare challenge type and opponent" framework event resolved. Since Crown Regent always takes place before any responses are triggered in the "initiate challenge/declare attackers" framework window, such responses will always look at the "new" defender in that situation.
Example: If A attacks B and B redirects to C (must be before any responses are triggered), B could NOT trigger his Euron/Bringers of Law, but C could.
The only place where things get kind of tricky is with the "redirect Shadows Varys." That's because the redirect is a response to initiating the challenge - just like all the other responses you are describing. So the following actually IS possible with Varys (assume B is First Player):
1) A attacks B.
2) B triggers the response on his Euron (at the time the response is triggered, he is the "against you" defending player).
3) C triggers Varys to redirect the challenge to himself.
4) A passes.
5) B passes.
6) C triggers the response on his Bringers of Law (at the time the response is triggered, HE is the "against you" defending player because of Varys).
Notice that none of this has anything to do with "after this character is declared as an attacker" responses (like Conclave) because redirects do not change or affect the attacking characters in any way.
Edited by ktom, 18 June 2014 - 08:37 AM.