1) Olivia Goodspeed dies from her pregnancy. That is why Horace is overly distraught when it's almost time for the baby to be born. This is also why he looks so sad when it's time to hear the "bad news" from Sawyer.

2) Charlotte's birthday is not a writing error. Said to be born in 1979 by Ben's review of her dossier could be true because he doesn't know where she was actually born. Meaning she could have possibly been born in 1971 (assuming that she's maybe 2-3 years old in 1974), however her birth certificate bear some sort of false information considering that her place of birth would be altered to cover the fact she was born on the island and 1979 may have been the new birthdate - representing the evacuation from the island. Her birth certificate may have been altered later in her life (possibly Widmore's group) in order to be able to dismiss the discrepancy in age difference (perhaps a time where she could pass for being 7-8 years younger). Remember, Ben stated that he was born on the Island to Kate when actually he got there when he was 10.

3) In case in may have not occured to you: Charlotte "disappeared" So as to not cause a time travel paradox. Two Charlottes cannot occupy the same time and space, so her child state stayed unchanged (because it a natural purpose for being there as opposed to her adult self) while her adult self disappeared.

4) Amy can have a normal pregnancy because Horace and Amy can leave the Island via submarine in order to go on a "vacation" off the Island and possibly get pregnant on Tahiti.

5) Losties may be secretly working for the Hostiles undercover as does Ben, so they later join the Hostiles and contribute to the Purge. I think 70's Ben will have a strong, manipultave control over the Losties. The same type of control and influence that impresses the Hostiles and what makes him "special".