If a monster attacks, at the end of the Damage Step: Its controller gains 300 LP. All monsters must attack, if able. During the End Phase: Destroy all face-up Attack Position monsters the turn player controls, that did not declare an attack.
Uh, well, considering no monsters are capable of attacking in the End Phase, duh...
More seriously, not sure why you think whether they were able to attack or not matters. If it's in Attack Position when Colosseum's effect resolves and didn't attack that turn, it dies. Pretty simple effect.
Hold on, how about this in an Aromage synchro with Ancient Fairy Dragon??
why without Island?
use island get incarnate replace island w this and get ultimate nightmare which must be attacked, only harm can be some instant removal on nightmare so probably run more revival traps and its even mor Kog than before