Did NEM start with a pure POS when it launched? If so, how did it achieve fair distribution?
NEM doesn't use POS nor POW.
NEM has it's one unique consensus algorithm called POI = Proof of Importance.
unlike to most POS coins there is no inflation.
all coins were created in the genesis block and you can only be rewarded with transaction fees.
POI means the more "important" (uptime of your node, number of transactions, ...) you are for the network the more you get.