1.4 Opel corsa idles erratic and engine service warning light comes on

Disconnect the battery negative cable (never the positive!) for 5 minutes

Sounds like a low fuel pressure issue or a stuck open egr valve/air leak.

The cause of high idle are vacuum leaks or a bad or dirty Idle air control. Check pcv valve to see if its working properly. pinch off the hose that supplies the PCV valve. The engine speed should drop slightly. If the engine returns to a normal idle speed, the PCV valve should be replaced. The Idle air control regulates engine rpms.

check your spark plugs wells for oil in them. also check for manifold air leaks spray some carb cleaner on the manifold and lister for engine change depending on the cleaner it will speed up on shut the engine down. also do this to air lines. any oil or water in them clean out if oil the seal on the cam cover is leaking.

Yes, it can be completely normal, engine idling RPM's will have various factors that affect them such as temperature, altitude, oil level. In all cars the RPM's will change when the compressor for the AC is activated.

sounds to me that you have a vacume leak on the engine.

idle control valve may be blocked. sits on the back of the intake carb. also lose intake top can give erratic ideling

The engine coolant temperature is high. Your other Codes are for missing cylinders on cylinder 1,2,3,and 4. Get the Codes cleared and start over. Changing the plugs and wires,etc could have generated the missing cylinder Codes.

Autozone,Advance, and Oreilys will scan for free.

Check your engine for an IDLE SPEED CONTROL SOLENOID, which may be in the intake. Or a fuel pressure regulator.

A lot of times people work on the spark side and forget the fuel injectors. They can cause a miss.