Ice and water shield should always be applied in the valleys and around penetrations before installing the new roof. This self adhered membrane seals around nails and helps to ensure a  long lasting project.

The pictures in this photo is a porch similar to our project house. You can see there are several facets and details involved, as well as extra ridge cap shingles and flashing materials.

In these photos, you can see hell a ridge vent works. The gap is Kurt in the decking at the ridge of the structure to allow hot air to escape at the highest points of the attic. A hard plastic ridge vent and ridge cap shingles are applied over the gap to allow are you super heated air to escape while preventing leaks. 

In this photo, you can see how are you sheathing is applied over existing slat boards with gaps. This is a very common practice and much less invasive than removing the old boards. In your case, it also provides a way to leave the original materials intact.