No, Deepcosmos. The mixed conditional you are thinking about combines unreal past (referring to the present) with conditional perfect (referring to the past), for example:
- There are no vampires here. If there was (were) one (actually, there are none), we would have already seen it.
In the original sentence, "came" and "bit" express real past. The past perfect "had come" and "had bitten" express hypothetical past. The past simple is not a simplified version of the past perfect in this case but expresses a different idea. However, combined with the conditional perfect, the result is more or less the same:
- If the first vampire came into existence that day and bit one person a month (if it is true that this happened), there would have been two vampires by Feb. 1, 1600.
- If the first vampire had come into existence that day and bitten one person a month (which was not the case), there would have been two vampires by Feb. 1, 1600.