It depends entirely on where the skateboards and other wood-derived waste ends up. If it's not properly disposed of (e.g. litter), then it will release most/all of it into the atmosphere. If it's incinerated, it's all released. If it's in a landfill, it depends on whether and how effectively the landfill is sealed.

Probably not. Most of the oil we find comes from a period when very few organisms could break down waste from dead trees and plants.