1. Yes, with the help of squidguard.
2. Yes, but make sure you run IIS in HTTP mode, not HTTPS. WPAD directions for pfSense.
3. Not that I am aware of.
4. Never had this happen to me, but in squid's config options you need to check Disable VIA Header and set the X-Forwarded Header Mode to delete. Without these changes, you are literally announcing to the world that you are proxied.