How do I remove the confirmation alert when choosing Silent mode?
This appears to be baked in to the stock ROM you are using. 

There are widgets, like silentwidget which will let you toggle the silent mode without using the power button method, although this requires you to go to the launcher to do this - to get around this, have a look at a multitasking app like PowerStrip which will allow you to launch apps, see widgets and more from anywhere on your device without having to go back to the launcher.