The man convicted of kidnapping her, now seriously ill in a prison medical center, has finally confessed to strangling her and dumping her remains along Interstate 89.

Does the preposition along suggest multiple points at the highway and the implication is that she had been dismembered?

Thanks in advance!
No. "Remains" in this context is her body - what remains of her after death [ie her soul/spirit has departed]. It does not indicate dismemberment.
Along always gives me the impression of multiple points along the journey.

Along the way to Yellowstone, the scenery was magnificent.

My friend was so drunk, he puked out of the car window along the way home. (Did my friend puke one or several times?)