Seriously, pack every available space inside the fuselage with C4, and when someone "captures" one of your drones, wait a couple of hours for them to put it somewhere sensitive... and blow it up. "What? Not our fault you guys mis-handled our equipment.":
Minimal. NTP packets are about 128 bytes. Individual clients will (if up to spec) contact no more than every 64 seconds, but up to 17 minutes once synchronized (or longer if using SNTP). I'm in the pool and I never notice the traffic.
Nope. Anyone with a stable time server is encouraged to join. The operative word being "stable". It's more about providing something that will be reliably *there* when it's needed. The protocol itself will take care of accuracy.