Firstly.... He was 13 when he killed his clan, he had just became a anbu captain, many had 3 tomoe sharingan, did you not see when the guys came to question itachi?, 2ndly there were others who had ms, it's been hinted by itachi and tobi, he said "Many have tried and failed to get the mangekyou sharingan" that doesnt mean that in the history of the uchiha only 5 people got it...., itachi also told kakashi "Let me show you, the reason why the uchiha were feared as the strongest clan, the power a true heir of the sharingan can obtain" once again, he makes it sound like there have been more people..... it doesnt seem that hard to unlock tbh, just see someone die , kakashi,shishui,itachi,izuna,madara,sasuke all got it, and thats most the uchiha we knew...
11 year old itachi was already extremely powerfull.. enough to beat a kage level sannin, 13 year old itachi got promoted to a anbu captain, further proving he was no ordinary ninja, stop underrating itachi