Actually, that one IS a pronunciation shift -- consider the epithet "Zeus Pater". Though in that case it's probably divergence from an ancestral religion that preceded both of them rather than syncretism.
Same likely goes for Dis Pater and the word Deus.
As I understand it, proto-Indo-European religion has the sky god
Dyeus Pitar, and the former word also gets use as a general term for gods/divinities (Sanskrit and Avestan/Iranian 'devas', Germanic 'tiwar'). The term Zeus comes from Dyeus, and Jupiter comes a transformation of the whole phrase. The 'modern' figure of Zeus also syncretizes the proto-Indo-European thunder god
Perkwunos, as shown by one of his epithets 'Zeus Keraunos'.
The thunder god most famously exists in a more modern form as Thor, but Indra might also be relatively well-known (though they share the same roles rather than clear etymological connections). In Norse/Germanic traditions, Dyeus Pitar seems to have become Tiwaz, who's now better known as Tyr, so Odin wasn't always head of the Germanic pantheon (maneuvering himself into power is quite in-character for him, symbolically).
If the Sheeda invasion increased the amount of people willing to deal with Hell in order for protection, I feel like people should also have reached out or be reaching out to less actively malevolent forces with genuine pleas and prayers out of desperation. Folkloric customs and superstitions in a lot of Christianized places often have roots in pre-Christian religious practices; Ireland is a prominent example of this, even if the Fair Folk are their own mix of problems (but there would have been something poetic about Sidhe vs. Sheeda being shown). If Morgaine le Fey somehow used the situation to her advantage by coming to Britain's aid and then proclaiming herself monarch, and there not being the military resources left to contest her, that could have been interesting. I don't know how people in Kahndaq fared with Adom, Isis and Osiris around, but I can't imagine he would be displeased with a revival of older Egyptian practice.
I imagine Hell still gets a lot of traffic because it's just more widely known about, the natural consequence of the influence of Christianity, especially after the Fawcett City incident. After the Silver City though, it would have been nice to see the angels actually acting positively/tangibly upon the world in response to people's prayers. I wonder if it's a somewhat similar situation as the Guardians' view of things, in that the Sheeda are technically human and so the whole thing is an internal human affair that angels can't officially interfere with (or the Congregation's existence is as far as they/God are going, but it's hard to gauge their global level of effectiveness). But still, there are plenty of non-Christian entities that still have sufficient fame for people to know about and plausibly pray to, and it would have been nice to see some story consequences of Thana's evangelism, which I know I keep going on about, and see a positive side of Zeus for once as a protector and defender of human civilization (which wouldn't excuse his dickish behavior but would at least make things more complicated and interesting besides the idea that he's just too powerful to be deposed. As things stand I kind of hope things are working towards him being deposed by the Paragon eventually, directly or indirectly through a faction of gods favorably inclined and empowered by him). I just remembered Hera should have been around acting like a human television host or something, so what did she do when the Sheeda were invading? Zagreus had also made a commitment to aiding humanity after his conversation with Paragon, though I don't remember all the details of that.