Exactly saying about 死に神, we can't say 死に神 is a mistake.
But in fact usually we use 死神.
Ａｌｌ of your examples of 死に神 were the special cases that authors intentionally made the sense of strangeness, old-fashioness, or a defference from usual 死神.
I bet that if a nonnative japanese speaker writes 死に神, most japanese think it is short of knowledge in 送り仮名(okurigana), a conjugational ending added in kana after a Chinese character.
Did I give you cooperation in understanding?
