The mirror is sensitive, easy to scratch with a cloth, I wouldn't worry about it.
Same for the focussing screen, that doesn't affect the photo either.

The sensor is behind the shutter, and you need to use the sensor cleaning menu option to get at it - at least that's how I do it on my Canon.
Unless you are silling to accept perhaps having to replace the sensor, don't try to clean it in any way other than blowing air in with a hand blower.

Don't use compressed air.
Hold the camera with lens mount down and blow air up into the body.

I had a problem on my 1D where even using the hand blower it fired a HUGE blob of grot into the camera that stuck to sensor and wouldn't come off.
It's going to go in to Canon for a wash ans brush up.