This will come with the barrel nut and everything else need to install. Because the KAK "Flash Can" attaches to the end of the barrel and because the barrel should always extend past the handguard, this handguard should not cause any issues when used with the "Flash Can."
4 inches total.
Please reach out to the manufacturer for an answer to your question because they do not make that information available.
No, the barrel nut is included with the handguard.
No, it is not.
Stern does not list these specifications, please contact them directly for further assistance.