My best guesses:
1) No idea. This would be a good place to test with a VM.
2) Depends on the options set when creating the CCP pkg. The default is to ignore conflicting processes, so it likely would encounter problems if the targeted software to upgrade is running when the pkg is applied.
3) The logs are a bit scattered, since the installation logs are per-program (e.g. InDesign, Illustrator, etc) plus the log of the master installation process itself. Take a look at Troubleshoot install issues with log files | CC.
4) If you mean you have a pkg with only updates included for an application where the original application isn't on the target machine (say, you have an update pkg that includes an update for Photoshop CC 2014.2.1 but not Photoshop itself, at least the Photoshop update would fail. Maybe the whole pkg. Consider packaging updates individually?
Regarding reenabling user-managed updates, see Disable auto-updates | Application Manager | IT administrators. That'll point you to the right file, at least it was the right file in CS6.