Yes. The KAK Micro 5/8-24 Slimline Flash Can will support all calibers with a bullet diameter of 0.458" or less. 6.5 Grendel has a bullet diameter of 0.264", so as long as you have 5/8x24 threads, this flash can will fit and function as intended.
The length stated is the overall length.
Yes, this should work for .300 AAC Blackout.
Unfortunately KAK only mentions that it is "THREADED 5/8-24 FOR 30 CALIBER OR SIMILAR WEAPONS", but they do not provide the exact exit diameter so it may need to be bored out to accommodate a .45 bullet.
No, this would be the muzzle device instead of a traditional brake/flash hider.
Unfortunately KAK only mentions that it is "THREADED 5/8-24 FOR 30 CALIBER OR SIMILAR WEAPONS", but they do not provide the exact exit diameter so it may need to be bored out to accommodate a 9mm bullet as that has a diameter of .355.
It does not require one, but if you as the shooter wanted to time the Flash Can for whatever reason, you could use a crush washer.