With regards to the use of the Dreadclaw and Vindicators. My justification would be that they were stolen from another warband during one of the frequent conflicts that occur between Chaos worshippers? You could tie this in with the lack of mutation. Perhaps Tzeentch's bargain also means that he will trick those who work against his plans into travelling to Ankou to clash with your warband and your warriors are required to slaughter them to a man or he will remove the protection he grants them that protects them from mutation? Maybe your guys see mutation as a mark of shame personally inflicted on them by Tzeentch as punishment for failure - the tactics used could be how your warband has learned to decimate its enemies, a strike of merciless savagery and bloodlust designed to give the enemy no time to react to their attack and kill them as quickly as possible, leaving no survivors and leaving the battlefield littered with choice artefacts and weaponry, as well as vehicles and wargear to scavenge and strip from the mutilated bodies of the dead?