When the next batch of S9's become available and assuming that the price will still be 2100 USD (I am not intending on starting a debate on what the price will be), does Bitmain adjust the BTC amount to reflect the current exchange rate of BTC/USD?
Generally yes. BUT they only do it once per batch from what I've noticed (mind you I dont pay real close attention).
So they set the price when the batch is made available for purchase and the BTC cost stays the same (even if the BTC/USD price changes) until the next.
In the past there have been two ways Bitmain sets the price: USD (and BTC price floats according to the exchange rate) or BTC (and USD price floats).
S9 B1 price was set in USD and the BTC price was floating. The exchange rate is at the bottom of every page on the Bitmain website. It might be lagging from the usual Bitstamp/Coinbase/etc rates, often in Bitmain's favor

, but generally accurate.
How they will set B2 price is anybody's guess.