Is it permissible to wear a necklace inscribed with "طسم الرحمن عسق الله" and not remove it, even when entering the lavatory? And is treatment for jinn permissible in Islamic law?

Ruqyah is permissible with the Book of Allah, His names, and supplications free from polytheism (shirk). If the mentioned amulet contains only these things, then it is permissible. However, scholars have differed on the permissibility of hanging it, with the preponderant opinion being that it is not permissible. If it contains polytheistic supplications or non-Arabic names, then it is not permissible.

It is recommended for someone who wishes to enter the lavatory while carrying something containing the remembrance of Allah to set it aside.

Ruqyah against jinn is permissible if it is done with the Book of Allah, His names, or supplications free from polytheism. The Messenger of Allah, peace be upon him, permitted ruqyah as long as it does not involve shirk. Whoever practices impermissible ruqyah should be advised and guided. If they do not desist, they should be restrained.