Question 1. Yes there is an adapter for the Pioneer HU:
Just one of a few different ones out there.
Question 2. You can either go with the Opus (preferred but expensive) or use an inverter and a shutdown controller. A quick search on here will give the details for either.
Question 3. I use XP Pro and without doing any tweaking have a boot time of around 30 seconds. The OS is actually your choice. It is going to be no different than your desktop as far as how it works generally speaking. Tweak all you like. Another search will yield different choices here as well.
Question 4. You can mount an LCD in your double din opening. Just do some more searching here and Google and you will find many tutorials and pics.