Tha an geamhradh a' tighinn.
"Calum's coming" should have been accepted.
You could argue that "Calum 's coming" should be accepted, but "Calum's coming" means something completely different to me!!
Calum's coming = Calum is coming. It could potentially mean the coming of Calum, but that would be a much rarer usage.
If "The coming of Calum" was wanted as a translation, wouldn't the Gaelic have been something like "Teachd Chaluim" rather than "Tha Calum a' tighinn"?