•On to my second point. Sage mode. Because Minatos sage mode was easily beaten by Juubidara it has been underrated a lot in my opinion. Minato said that it takes him long to knead the chakra for it and that he has never used it in battle, therefore he had no practice with it until this point. Take a closer look at that chapter and one would see it actually did not take Minato that long to enter sage mode! He just takes longer to enter the transformation than lets say... people like Naruto and Hashirama who can enter it in about 3 seconds top.
According to the panel order, minato:
1. Throws his kunai at Madara's feet. (Movement)
2. Gaara builds a sand wall - meaning Minatos kunai would have had to have passed and landed before that wall was built.
3. Next panel shows minato just entering sage mode - the effects indicated by kishi shows that the sage pigments appearing on Minatos eyes at that given time. Meaning he couldn't have been gathering senjutsu until after he threw the kunai otherwise the movement would have broken the senjutsu gathering.
Consider the time between the kunai being thrown, the construction of the sand wall and Kakashi activating his MS plus the completion of the marks appearing around Minatos face. That's around 10 or more seconds.