That seems to me to be a sort of Greco-Latin hybrid in its declension. For Hermione should be (for any students of Latin who lack any Greek experience):
Hermione (nom)
Hermiones (gen)
Hermione (dat; iota subscript for the final eta in Koine, at some point earlier it was an actual iota)
Hermionen (acc)
Basing my observations only by what you've said (and therefore not knowing what is used for the genitive), I'd say the following:
He is keeping the nominative (of course).
He is changing the Greek "n" to a more Latin "m", perhaps to ease reading for those fairly new Latinists who haven't yet seen forms like Aenean (a frequent accusative of Aeneas) in print.
He is stretching the iota subscript in the dative to be changed to an actual i (somewhat like the Third), again in an effort to ease reading by newer readers.
Now, I think it would be smoother and more logical to simply use Hermione, -ae to both be most authentic and logical; but I can at least see what they're trying to do here. But of course, I don't know what they did with the genitive, which would really be the crux of it all.
Hermione (nom)
Hermiones (gen)
Hermione (dat; iota subscript for the final eta in Koine, at some point earlier it was an actual iota)
Hermionen (acc)
Basing my observations only by what you've said (and therefore not knowing what is used for the genitive), I'd say the following:
He is keeping the nominative (of course).
He is changing the Greek "n" to a more Latin "m", perhaps to ease reading for those fairly new Latinists who haven't yet seen forms like Aenean (a frequent accusative of Aeneas) in print.
He is stretching the iota subscript in the dative to be changed to an actual i (somewhat like the Third), again in an effort to ease reading by newer readers.
Now, I think it would be smoother and more logical to simply use Hermione, -ae to both be most authentic and logical; but I can at least see what they're trying to do here. But of course, I don't know what they did with the genitive, which would really be the crux of it all.