Q1: No. A has no relationship to C, regardless of B's alliance. A could attack C without a problem. Note that if B joins the battle to help his ally C, B will become the traitor against A.
Q2: This is a good question. You only need to have Dimplomatic relations with one player of an Alliance to join that Alliance, but players with the Traitor card cannot form new Dimplomatic relationships. So the real question is: Is upgrading Dimplomatic Relations to an Alliance considered a new Dimplomatic Relationship?
I believe that player A (a Traitor) would be able to join the Alliance. Since he had Dimplomatic Relations with E prior to becoming the Traitor, he would qualify to join the Alliance so long as both D&E agree.
Q2: This is a good question. You only need to have Dimplomatic relations with one player of an Alliance to join that Alliance, but players with the Traitor card cannot form new Dimplomatic relationships. So the real question is: Is upgrading Dimplomatic Relations to an Alliance considered a new Dimplomatic Relationship?
I believe that player A (a Traitor) would be able to join the Alliance. Since he had Dimplomatic Relations with E prior to becoming the Traitor, he would qualify to join the Alliance so long as both D&E agree.