What distinguishes an interpolation from an acceptable addition by a reliable narrator?

Interpolated, Reversed, Extra-Link, Inconsistent, and Textually Altered Reports

• Both necessarily involve lying
• A reliable addition always occurs at the end
• Every interpolation is ṣaḥīḥ li-dhātihi
• Idrāj intrudes nonoriginal material; a validated addition can be genuinely transmitted wording