You can use SD Maid 2/SE, it's a trustable application.

I don't think so he got enough time to cheat you in just 5 mins.Anyways you can check for scratches in back glass, dents on side frame which could have come because of back glass removal tool or maybe dust in your camera glass from inside can give yo...

Dont forget to stay in a cool/AC environment to avoid excessive heating and possible green line issues.

It's a display issue, the service centre will change the display and it will be fine.Not sure why it happens, the same happened with my watch 4 classic. (Mine was in warranty so it was done free of cost)

Yup, camera quality is worse as compared to One Ui 6. It's a lot more grainy now.