Hmm, interesting... Most English sources claim that the confluence with Kashkan (SW of Pol-e-Dokhtar) marks the beginning of the name Karkheh. But they may all have copied from the same wrong source. Do you have a written source that identifies the mouth of Zal as the right point?
Was Kerkheh mapped up to Kaskan on OSM in earlier times? This would explain why on https://commons.wikimedia.org/wiki/File:Seymareh_Landslide_01.png, the river is called Kerkheh, and the image author says that OSM is the source of the names. Otherwise, the landslide should have been named Kerkheh landslide then...
The problem is: The naming on wikipedia:EN and the complete category tree on wikicommons (and I have not checked wikidata yet) obviously reflect the wrong but internationally recepted system. If we try to make OSM consistent with WP etc (which I would prefer), then we should have VERY good (and published) argument to do so.