How authentic is the Hadith: "There is no god but Allah is My fortress; whoever enters My fortress is safe from My punishment"? And is it permissible to take knowledge from العلامة Al-Habib Umar bin Hafiz and Sheikh Al-Habib bin Ali Al-Jifri?

It is not within the site's purview to pass judgment on individuals. A Muslim should make people known by the truth, study the Qur'an and Sunnah, and only acquire knowledge from those known for their adherence to the Sunnah and following, and who are attested to by trustworthy scholars. As for the aforementioned hadith, it has been deemed weak by Al-Iraqi and Al-Albani.