I was under the impression 6th coffin was something that couldnt be activated until now. I was under the impression it was Rikudou Sennin and Kabuto couldnt activate it even he could summon it, it would made sense with his statement "I will unlock the mind of RS" which made me think perhaps all was a distraction while he was unlocking it.
However bar the concentration he needed to summon Madara, there is no reason he couldnt have used it from the beginning of the war and rape the alliance, he is appearing in a solo and still is going to rape the alliance only to be stopped at the last minute imagine if he was there when Akatsuki was at full strenght. I mean maybe because of the concentration needed he wouldnt had been summoned at the outright beginning but lets say at least after the Kin/Gin battle, or imagine instead of teaming Itachi with Nagato team Madara, Kabuto would had captured the jinchuurikis by now. This would had been a way more logical option, even if Madara is stronger than Itachi and Nagato team which should be as he is the trump card, his power would had been boosted to insane levels if he was teamed up with Nagato, imagine a guy stronger than Nagato + Itachi teamed up with Nagato, this would had been a more logical choice than team Itachi as Madara has a higher degree of Sharingan than him, it would had been the true team of ultimate doujutsus which was the reason for teaming Nagato and Itachi, imagine a guy with powers BEYOND EMS (I expect it to be since it is the trump card so possibly he has powers greater than EMS which he got after gaining EMS, maybe even higher than Rinnegan) and Nagato working together.
However bar the concentration he needed to summon Madara, there is no reason he couldnt have used it from the beginning of the war and rape the alliance, he is appearing in a solo and still is going to rape the alliance only to be stopped at the last minute imagine if he was there when Akatsuki was at full strenght. I mean maybe because of the concentration needed he wouldnt had been summoned at the outright beginning but lets say at least after the Kin/Gin battle, or imagine instead of teaming Itachi with Nagato team Madara, Kabuto would had captured the jinchuurikis by now. This would had been a way more logical option, even if Madara is stronger than Itachi and Nagato team which should be as he is the trump card, his power would had been boosted to insane levels if he was teamed up with Nagato, imagine a guy stronger than Nagato + Itachi teamed up with Nagato, this would had been a more logical choice than team Itachi as Madara has a higher degree of Sharingan than him, it would had been the true team of ultimate doujutsus which was the reason for teaming Nagato and Itachi, imagine a guy with powers BEYOND EMS (I expect it to be since it is the trump card so possibly he has powers greater than EMS which he got after gaining EMS, maybe even higher than Rinnegan) and Nagato working together.