2. What type of slices are there?
The three basic types of slices.
Which problem to correct first? Swing path or Clubface?.
You will have much greater success working to control the swing path first, because you can measure your progress be viewing your divots or scuff marks on the practice tee. Also you may find the faults that cause the outside-to-inside swing path also create your open clubface position.
The outside-to-inside swing is the biggest obstacle to playing good golf that most golfers will ever face. With an outside-to-inside swing, if the clubface is square to the swing path you will have a pull. This is a shot that goes straight left. If on the outside-to-inside swing the clubface is closed (aimed to the left of the swing path), you will have a pull hook that starts to the left and then hooks further left.
Finally, if the clubface is open (aimed to the right of the swing path) you will have a pull slice that starts to the left and then slices back to the right. So when you have an outside-to-inside swing path you can actually be hitting a straight shot (yes, straight left) or a slice or a hook, all of which start to the left. Work on correcting only one fault at a time or you will become confused and frustrated. After you correct your outside-to-inside swing, you may still slice because of the open clubface. Then when you work on squaring the clubface your slice will be gone.