^^ Oh so you mean that Kabuto's ace would be Kushina for he has her body? So Naruto is the only ninja who can kill Madara right?...

^^ In that case Naruto would become Rikudo in some other part of the series, hope that'll be in an epic way..:) Hehehehe, I want to see how Madara looks of he saw Naruto to be a Rikudo..:)

Madara's body composition being similar to that of the White Zetsu was confirmed this chapter by Tsunade:

It was also confirmed this chapter that Naruto's attacks make Shodai's cells go out of control and transform the person into a plant:

So Minato obviously knew that Madara's immortality came from Shodai's cells and thus he sealed the Kyuubi into Naruto so that his chakra would develop the life inducing element that would cause Shodai's cells to transform into plants.

He also left the perfect attack in the form of Rasengan to be passed down to Naruto, that can pack enough life energy into a single attack that it would overwhelm Madara's will and cause him to lose control of Shodai's cells in the same way as danzo did.

This also explains why Minato only sealed the Yang half of the Kyuubi in Naruto, since Naruto only need the Yang half as the life energy source and his own Uzumaki features.

So its possible that Kabuto's 6th Coffin is Uzamaki Mito or Kushina and who also had the same life inducing chakra and thats why Madara is afraid of the 6th Coffin since Mito/Kushina is the only one who can kill him since Naruto and Kabuto figured this out through his research of Shodai Hokage's DNA and thats why Madara calls him a shrewd man.

When did she got Madara's Body sample to examine?

Then why not Yamato turned into a Tree?

When did Minato knew that Masked Man Madara is using First Hokage's Cell. Till now Minato knew him as Masked Man. He could have told to Naruto about Madara if he knew

If Naruto would not have known about Rasengan he could have used Tailed Beast Bomb at least learned from Bee. He didn't left that Rasengan for Naruto, rather Jiraiya used that technique to pass to his son.

Who can predict about someone when the person is just born . Had Minato knew that Naruto will pass his childhood hated by the Villagers. He said the life will be difficult as Naruto's parents were not there to care about born Naruto.
Naruto could have got the Namikage Clan Abilities. He was given given Uzumaki title so that enemy can't think that he is the son of Fourth Hokage and capture him.

We saw Sakura examining a Zetsu at the beginning and then informing Tsunade about it.

He made a guess and it panned out since he realized that for Madara to actually be there he would need something to increase his life span and since he knew that Madara had fought Hashirama he speculated that had acquired his genetic material that would allow him to sustain his existence. Minato also said himself that he didn't have enough time to go into details.

Since Naruto cannot enter at the moment the Tailed Beast form he cannot use the Beast Bomb as the Hachibi told him. That is why Minato left the Rasengan because Naruto would learn it and the be able to produce the Beast Bomb in his current RM form.

That doesn't mean they got Madara's body to examine

Minato was said to be the strongest Hokage. The person who was defeated by the Fist Hokage should not stand a chance upon Minato. The ST Technique was not explained by anyone when describing Madara. Then how the Fourth realized it could be Madara none other than him. He told about Masked man not Madara to Naruto. On the other Hand, the person claiming Madara is Tobi.

Rasengan was developed by Minato when Naruto was not born. So has he seen the future that Madara will destroy Konoha and he will seal Kyuubi inside Naruto. As Rasengan is not an easy technique , It requires Time to develop it. Tailing the Technique might have taken time for Jiraiya also. Jiraiya was also official Rasengan user accepted by Tsunade in Search for Tsunade Arc

Actually, there's no way Minato knew about Madara's body composition and sealed the part of the Kyuubi which worked better for him inside Naruto.

Minato sealed only half the Kyuubi's chakra inside Naruto because:

1.He said he couldnt seal all of it
and
2. He clearly chose to seal the part which wont be as dangerous to Naruto, had it been the Yin chakra, Naruto would've been overwhelmed by hatred much easier. Not to mention that Naruto is half an Uzumaki.....

About Rasengan:

I honestly believe that Minato developed that technique for Kushina, but since the Rasengan is a great technique no matter how u slice it, its use became frequent. (Naruto getting it was coincidence imo, both Kakashi and Jiraiya use it.)

Actually, everyone with IQ higher than 89 can realize (especially after they failed to finish off the job) that Madara isnt ur ordinary enemy.
Minato knew that the Kyuubi was a powerful weapon, and using it against him is the only way to win, Hashirama did the exact same thing, he turned the Kyuubi against Madara.