Answer 1: Yes, you should add all 15 keys and uncheck the “Recurse Subfolders” flag. Based on my own experience, don’t leave this flag checked and don’t ask me why…
Answer 2: You won’t be able to save a template if a part used in one of the rule is missing. But if you managed to save it, which I am sure you won’t, compliance job will fail, with a RED X.
Answer 3: Parts get executed regardless if they exist on the target server. However during the rule execution there is when data are collected and checked (target server) to ensure they are complaint if they match the conditions or non-compliant otherwise.
I've seen errors thrown if you specify a part that does not exist on a target - so instead of choosing a local user account "Administrator" in my part I choose "Local Users" and in my rule i say something like "Administrator must exist and meet these conditions". this seems to keep me from getting errors during the 'collection' phase.