Your computer would fail to boot. I *think* you'll get a logon screen from Winlogon being started but that's it. Honestly, I'm not 100% sure because I've never tried such a ludicrous thing
Yes, a manual service can still load if something starts it, but what's going to start it?
Logical disk manager for example will get started by disk management when you go to configure your drives.
Services that are set to automatic are started by SMSS.exe during boot. The key is "during boot". SMSS.exe won't attempt to start them at some later time.
Edit: Correction. You should be able to boot provided only your automatic services, and not your automatic drivers are set to manual. You're not going to be able to do jack sh*t though. Be sure to either note everything you change, or make a backup of your registry before you begin. Last known good won't save you, and a system restore point will be unavailable.