1. There's a checkbox option in each scene' properties tab:
scroll scene if the cursor is at screen edge. If you tick that then the screen will scroll whenever the mouse enters the scroll borders at the edge of the current viewport.
2. Sure, but you have to disable character centering. You will have to re-enable character centering when you want to make the camera automatically follow the character again.
a. scene > center scene on character permanently > select character (tick clear centering checkbox)
b. now you can define a scroll position without it scrolling by creating an execute a script action part containing...
game.ScrollPosition = {x = 0, y = 0} -- replace 0, 0 with coordinates you want to set scroll position to.
To re-enable camera follow character, you just have to follow step A, but leave the clear centering checkbox unchecked.