When detectives Benson and Stabler interrogate the aunt during a spell of dementia, she recalls Hurricane Camille, not Katrina, as the most recent storm to hit New Orleans and identifies John F. Kennedy as the current President. Hurricane Camille occurred during 1969, when Richard Nixon was President.
Casey says they don't have any probable cause to arrest Alvin Dutch so they can't hold him. However they know he kidnapped Tasha and her two younger sisters, they have her sworn statement saying so, this is more than enough probable cause to arrest Alvin on three counts of first degree kidnapping of a minor. With Alvin being an out-of-state resident with no fixed address and no ties to the community being accused of kidnapping three little girls he almost certainly would of been remanded at arraignment or at the very least been given a very high bail amount.