• What is the ruling for using aḥād ḥadīths in the details of beliefs about the Hereafter? What is the ruling for someone who denies them?
• How would you respond to doubts people might have about he, may Allah bless him and grant him peace, sending singular individuals to teach people beliefs and rulings?
• What is the difference between making knowledge obligatory and acting upon it when one does not act unless it is after knowledge, as there can be no knowledge with ignorance? What does it mean that the Ummah has accepted the two Ṣaḥīḥ collections? What is meant and what does that necessitate? How does one refute the claims of Salafiyyah regarding this

One is obligated to affirm the contents of authentic āḥād ḥadīths in theological matters. However, one is not guilty of disbelief by denying them as long as the ḥadīth is āḥād and its contents do not reach the level of tawātur [i.e. being mass-transmitted]. Rather, one is iniquitous (fāsiq). A Muslim is not guilty of disbelief unless he denies that which is known of the religion by necessity.

As for the doubt you have mentioned regarding the singular individuals that the Messenger of Allah, may Allah bless him and grant him peace, sent to distant lands to teach people the doctrines of Islam, the object of conjecture (ẓann) is the ḥadīths of these individuals and their explanation of doctrinal matters. However, the Messenger of Allah, may Allah bless him and grant him peace, sending one of them, such as Muʿādh ibn Jabal, for example, to teach them Islam, is a matter of certainty. The foundation of the obligation to act upon Muʿādh’s teachings is the absolute certainty that he is the messenger of Allah’s Messenger, may Allah bless him and grant him peace, to them. The Messenger of Allah, may Allah bless him and grant him peace, would say, {Whenever a messenger comes to you from me to teach you the doctrines of Islam, you are obligated to act upon what he tells you.} Therefore, acting upon what the messenger of Allah’s Messenger, may Allah bless him and grant him peace, brings is like acting upon that which is mutawātir. (For further details, please refer to al-Musṭaṣfā by Ḥujjat al-Islam, Abū Ḥāmid al-Ghazālī, where he explains the difference between mutawātir and āḥād ḥadīths.) For a detailed answer to your other questions, you can find it in my book as-Salafiyyah.