Book in question: The Rise of Mahasena:The transformation of Skanda-Karttikeya in North India from the Kusana to Gupta Empires(Brill’s Indological Library) by Richard D Mann
The book in question is good regarding some early textual sources and also highlighing some of the grahas associated with Him,but I found it unsatisfactory in its explanation of why Skanda upāsanā has almost completely vanished from the North(claiming that ‘refining’ Him led to a loss of popularity and what not),while a more fuller picture of Him(presenting His tāttvic reality in the language of siddhānta āgama rather than the older Kaumāra tantra as we see in Arunagirinathar’s hymns) long after that form of upAsanA was forgotten in the North. As @Ghorangirasa noted: Blaming a largely southern “refinement” for the loss of popularity in the north is ill-conceived. The book however makes one decent point:That he was subsumed into [Saiddhāntika] Śaiva mārga,though I would place this subsuming somewhat later. One can say that this subsuming has its beginnings in the Gupta era,nothing more than that.
That being said,Skanda and Vishnu are two deities whose mantramārga(not in terms of mantras standalone,but the full scale dīkṣā etc) have been subsumed into contexts distinct from their original context and the original tāntrika picture of these deities(talking of archaic Pāñcarātra for Vishnu and Kaumāra tantra for skanda) has faded/been co-opted into other sampradāyas(Śrīvaiṣṇava and saiddhāntika śaiva) and have become more ‘bhaktimārga pradhāna’,for lack of a better word.
OM agninandanāya namaḥ|