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Separate names with a comma.
As stated on the resource page, Versions lower than 1.8.8 are not supported and there are no plans to ever support them
Stuff like "hidden" values within the CraftBukkit implementation that the Bukkit API doesn't expose by default, packet listening, etc.
No - no functioning anticheat can be made with Skript, it simply isn't performant enough and doesn't give you access to the things you need in...
I think you may be mistaken, the method source prevents the returned object from ever being null, it always returns an OfflinePlayer.