DTC's must to diagnosed and repaired in the order that they appear. Then after the repair, clear all DTC's from the memory, and perform a drive cycle to confirm the repair. Disconnecting the battery will turn the SES light off provided there are no hard faults. It will not remove the DTC's from memory, that must be done with a scan tool.
Whatever caused the P0300 to set, likely caused the other DTC's to set also. After diagnosing, repairing the cause of the P0300, clearing the memory, and performing a drive cycle, it's quite possible the other DTC's will not return.
P0300 is almost always caused by a fault in the secondary ignition system: Spark plugs, spark plug wires, distributor cap & rotor, and ignition coil however, it can also be caused by low fuel pressure.