Will the testimony of "There is no god but Allah" be accepted from someone who has pondered and uttered thoughts involving doubts about the existence of Allah, the Day of Judgment, and the authenticity of the noble Prophetic traditions? Is this considered apostasy from Islam? And is it obligatory to inform one's parents or future spouse about sins of lesbianism and fornication?

Lesbianism is forbidden, but what is said about "La ilaha illa Allah" not being accepted because of it is incorrect. Repentance, with its conditions, is accepted. The hadith mentioned in this regard is fabricated. Adultery with a brother is a greater sin, and you have done well by repenting. You must be careful not to return to it. Treat your brother as a stranger if you see anything suspicious from him. Do not tell your parents what you did, because a Muslim is required to conceal their own sins. You are not obliged to tell a suitor about what happened regarding adultery or lesbianism. As for negative thoughts about Allah and the Day of Judgment, do not pay attention to them. Your hatred for them is a sign of faith, and you must seek refuge with Allah from Satan.