Is the Dawah's handling of the brother who has leaned towards Salafi thought sound, and is it permissible for him to disclose the Dawah's secrets after having sworn an oath, especially given the existence of a personal dispute and the distortion of facts?

A Muslim must work for Islam seeking the countenance of Allah and establishing His call, and cooperate with his fellow callers to righteousness and piety. He should advise them, debate with them kindly, and love them for the sake of Allah. He must also think well of his brethren, be sincere in his intention to Allah in cooperating with them, and not divulge their secrets entrusted to him.

As for inclining towards the Salafi methodology, it does not prevent collective work as long as it does not conflict with the group's objectives or seek to destabilize its organization. Obedience to those in charge is obligatory in what is good, and those in charge have the right to relieve an individual from responsibility if they deem it beneficial for the work.