Is a person considered an apostate if they enter a state of doubt about Islam due to suspicions raised by others? What is the way to return to certainty and deeper understanding of the religion? And would re-uttering the Two Testimonies benefit them?

Waswasah (whispers/obsessive thoughts) is what assails the heart without conscious choice, and disliking it is a clear sign of faith. As for doubt, it can invalidate the covenant of Islam if it takes root in the heart, because it contradicts the heart's declaration, which is belief. Apostasy can occur through speech, action, belief, and doubt, and doubt is the opposite of certainty. A servant cannot be a believer except with firm certainty, for Allah says: "The believers are only those who have believed in Allah and His Messenger and thereafter doubt not," and the Prophet (peace be upon him) said: "I bear witness that there is no god but Allah and that I am the Messenger of Allah. No servant meets Allah with these two [testimonies], not doubting in them, except that he will enter Paradise." Doubt may occur to a person while inwardly they are believing, and the repentance of one who doubts is to utter the two testimonies with certainty regarding that which was doubted. To attain certainty, one must reflect upon the signs of the Prophet's truthfulness (peace be upon him), ponder over the words of Allah, read scholarly books on the miraculous nature of the Quran, seek beneficial knowledge, and associate with righteous people.