It sounds like it needs tuning like a fine violin needs tuning to play a beautiful tune
Typically this is caused by the load being removed from the engine, allowing it to rev faster and thus idle higher.
rapidly changing gears in anything will hurt the vechicle but not slowing down will not
the clutch pedal
You may have wear in your transmission causing line pressure too be to high, or you may have broken mounts.
No, that has nothing to do with idle speed.
no this will not ruin your car BUT it will start to scrape your gear because it is not being shifted properly
Clutch pedal
Gearbox oil needs changing.
A transmission that causes the RPMs to increase without changing gears could be in need of fluid. If that does not fix the problem, it may be the car needs a new transmission. For safety reasons, it is imperative the car is taken to a trained mechanic to be diagnosed properly.
The pedal on the left is the clutch. You press it when you are changing gears.
Yes if it is stuck open.
You do not have the correct timing set for that car.