Can jinn or devils be present in a mirror, can they be seen through it, is it permissible to write the Al-Isti‘adha (seeking refuge with Allah) on it, is it permissible to supplicate while looking into it, and does doubting these matters constitute shirk (associating partners with Allah) or disbelief?

There is no evidence for the presence of a devil or jinn in a mirror, and it is Islamically permissible for a human to see a jinn. It is not obligatory to write "Audhu billah" (I seek refuge in Allah) on a mirror, and it is permissible to supplicate while looking into it. Doubting the presence of jinn in a mirror is not considered shirk (polytheism), and denying the possibility of seeing jinn is neither shirk nor apostasy, even though it is Islamically and rationally permissible. The verse: Indeed, he sees you, he and his tribe, from where you do not see them, indicates that the default is that jinn see us, and we do not see them, but it does not absolutely negate the possibility of us seeing them.