No, can't.
Usually an AR15 5.56 barrel thread pitch is 1/2-28. The muzzle thread pitch is same as the barrel.
If your barrel thread pitch is different with this thread pitch, you have to choose other muzzle.
The OD is 0.98"
Yes, you would need the .308 models with the 5/8x24 thread pitch: 20Q-BMB-MCRECOM-ASMZ06
Not sure what you mean, The gas block will need to be installed before the muzzle device and if you are trying to switch gas blocks, you will need to remove the muzzle device.
I would recommend contacting the manufacture.
Please email us at techsales@opticsplanet.com so we can further answer your question.
Yes, it will work just fine.
Yes, as long as the 6.5 Creedmoor barrel is 5/8x24 threaded as well.
Any of the following options should work for you:
https://www.opticsplanet.com/guntec-usa-gen-2-steel-multi-port-compensator.html?_iv_caliber-193146313=dot-450
https://www.opticsplanet.com/glfa-muzzle-brake-devastator-450bushmstr-11-16-x24-s-s.html
https://www.opticsplanet.com/glfa-muzzle-brake-devastator-450bushmstr-11-16x24-black.html
https://www.opticsplanet.com/glfa-muzzle-brake-stainless-450-bushmaster-11-16-x24.html
https://www.opticsplanet.com/glfa-muzzle-brake-450-bushmaster-11-16-x24-threads.html