2004 Maxima won't start after battery was disconnected for 5 days while tranny was being replaced. Turns over fine and dies. I get a P1615 NATS Malfunction code (security system issue) and a P0335 Crankshift sensor circuit problem. Is the 1615 issue causing the P0335 issue or does the crankshaft position sensor need to be replaced?

Not so silly.
While almost anything can happen when the power is taken away from vital electronics for an extended period, in your particular situation, I don't think these codes are related. With an unrecognized key trying to start the car, you shouldn't even be able to crank the engine over. Don't replace the sensor until the other fault is fixed.

The P0335 relates to the Crankshaft Position Sensor (CKP) and the P1615 is in regards to an uninitialized ignition key, something that may occur due to extended power-off.

Have you consuted your local dealer about having the key's tested and reprogrammed? They are likely to be the only shop to have the appropriate tools to do this.