Puranas are not considered as supreme authorities. They belong to the class of Scriptures called Smriti. The authentic scriptures are the Vedas, Upanishads, Gita. They are regarded as Shruti. My own impression of Garuda purana is that it was possibly written with the idea of instilling fear in the minds of people, possibly under Christian and Islamic influence, to prevent them from going astray. But as you rightly surmise these things hardly help.
As to the Katha Upanishad, it is a Shruti so carries its own authority. Its theme is the discovery of the secret of immortality and death is only a means and occasion to reveal the secret.
Affectionately,
Alok Da
