In 'Tamizh Ilakkiya varalaaru' by Mu.Va, the author talks of the references to MUrugan worship and maintains that the Somaskanda concept came into being during the first six centuries of Modern era. As for Adi Shankara's references, I think there is still a controversy over his dates itself. He can be parked anywhere from the 5th century BC(if you take Kanchi Madam's dating) to sixth century CE (in line with the Bhakthi movement). Thus I dont think trying to date the Somaskanda worship with AdiShankara's help is not going to fly far. Furter, if you take the datings of Neelakanda Sastri et al, the Sangam age itself is the first 2-3 centuries of the common era and Gokul's thought that the integration between Kottravai and Shiva occured during this period will fit well with the Shaivist revival and the Bhakthi movement that followed this period. As far as I know, Sangam literature has lot of references to Murugan worship (especially whenever the Vettuvars are mentioned and Kabilar has sung on the same also) but no mention of Murugan being the son of Shiva. I, for one, believe that Murugan along with the Naattar deivangal that followed are the Tamil Gods who can be traced back to the origins of the language itself. But as I mentioned it is a belief and there are factual proofs needed for substantiating it.
And I am not a scholar like Gokul but I like the way things connect between the various things I've read. Point out any mistakes in the above.