Then, Black Zetsu would have explicitly say, that, after Madara obtained a piece of Hashirama's flesh, he was able to perform Izanagi. But no, he explicitly noted that Madara created Izanagi in his eye. And again, having a piece of Hashirama's meat in the stomach doesn't equal having the DNA. After he was revived, he implanted those cells. It's a weak excuse from your side. Madara certainly used Izanagi without Hashirama's DNA involved, hence Black Zetsu phrased it this way. And as for Onoki, Jinton is an entirely other matter. It's size is depended on the Chakra put in, as the larger the amount of Chakra put in, the larger the three dimensional object becomes, or the more power it releases. All the Jinton is based on, is the creation of the same three dimensional objects which merely vary in size and form. If a Suiton user on the other hand wanted to use a larger Suiton technique, he would rather insist on another kind of Ninjutsu which would provide said scale. In other words, if Madara wants to use a huger Katon, he rather uses Gōka Messhitsu instead scaling his initial Katon Jutsu up with more Chakra.
Even with a boost of Tsunade, Mei's Suiton wouldn't come near Juubi's size, which has the size of a mountain.
I think it's also not debatable, that Hashirama's DNA did not boost Madara's Katon. Madara is the most powerful Katon user ever, so those powerful Katon Jutsu are reduced to his skills he was feared for and for which Tsunade warned, emphasizing the feats he showed. Hashirama's DNA didn't boost Edo Madara's other techniques either. So these are empty words.
2) Obito noted that, in order to open other dimensions than his own, he would require a huge amount of Chakra. Chakra which Sakura possessed ue to storing up Chakra in her Byakugo seal for years, something probably not even Tsunade did. So if anything, Sakura is an advanced version of Tsunade.
3) The auto-defence of Gaara's sand doesn't matter in this case, as Genjutsu only requires visual contact with no movements the sand should react to. Apart from that, I already pointed out that with Izanagi, the used determines the destination of his revival. If he appears right behind the opponents back, the sand won't react as to an incoming attack.