Something that I think underscores your question is a problem in our understandings of the prevalence of and adherence to Christianity in the 4th century. The simple problem is that we don't really know how many people converted to Christianity in the wake of Constantine, and our best guesses are super problematic. Rodney Stark's (now quite old) estimates are the ones that I see most frequently quoted (table reproduced below), but he uses a lot of problematic methods to calculate these numbers and I don't usually put much stock into them. At the same time, the numbers probably aren't too far off the mark (if you count a likely ±20% off as close, especially for the later numbers), and they paint a fairly clear picture: the Roman Empire was not entirely Christian by the middle of the 4th century, and we have tons of evidence of pagan ritual continuing far beyond 350, even in the presence of harsh anti-pagan laws promulgated by Constantine and his heirs (laws in the Roman period are often more indicative as evidence of a prohibited practice occurring than as evidence of adequate enforcement).

Year	Number of Christians	% of Roman Population
40	1,000	0.0017
200	217,795	0.36
300	6,288,832	10.5
350	33,882,008	56.5

Stark, Rodney. The Rise of Christianity: A Sociologist Reconsiders History (Princeton University Press, 1996)

Basically, what this means is that there were a lot of followers of the traditional Roman religion throughout the reign of Constantius II. Even among those who had converted to Christianity, belief systems were in no way standardized or always follow our understanding of ancient Christian practices. Well into the late 4th c., Christian religious figures like John Chrysostom (349-407; Deacon in the Church of Antioch, later Patriarch of Constantinople) rail against self-described Christians practicing pagan and Jewish rituals, the implication being that many normal Romans didn't fully grasp the degree to which Christianity was an exclusive religion or that continuing other practices was prohibited or against the religion. In summary, in the mid-4th century, not only were there a lot of pagans left around, but many Christians did not follow a "pure" form of Christianity and retained older non-Christian practices.

On the deification front, traditional Roman religious practice, since Augustus, had placed the Emperor in a particularly central place in the practice of ritual and religion. Deification was a sign of respect and fidelity to an Emperor or Imperial Dynasty that also had political and social implications. Even after legalizing Christianity, Constantine was the target of deification by at least one municipal government. The so-called Hispellum Rescript shows that Constantine granted the city of Hispellum (modern Spello in Umbria, Italy) a temple to be built for the gens Flavia (basically his family/dynasty), and the city itself was named Flavia Constans in his honor. As long as the inhabitants did not sacrifice to him (sacrifice being one of the the main points of contention in Christian-pagan relations), Constantine seemed to be not too bothered by the idea that a group of pagans would be praying to his family. Thus, when the Roman Senate deified Constantius II at his death, they were simply following the traditions of the Roman state, where the Emperor was the head of Roman religion.