No anime didn't have him use seals. I wasn't trying to say he did or not use seal/jutsu.

I was just pointing out that Danzou had to use seals to use Izanagi ( Canon manga materials) while based on what i remember Tobi didn't. And yes this could very well be one of the differences between a perfect vs imperfect Izanagis. But thus far this is just speculations and is not confirmed.

So i hope Kishi clear this part and confirm for us how Izangai actually works. So far he hasn't been clear enough imo.

I also wrote about him being the most probable candidate much for the same reasons that you listed. But i dropped that idea after Madara stated in his fight against the 5 kages that his brother is dead. Or was it a mistranslation? I mean Madara had Tobi as his helper, so he would have to know that Tobi is Izuna if they were working together.

Could it be that he used Zetsu? We have already seen Zetsu spying successfully on sharingan users like Itachi and byakugan user at the kage summit and many other sensing ninja couldn't tell who he was. We have seen him impersonating any ninja from the alliance army and no one was able to tell the difference, also Zetsu did this using multiple bodies at the same time, he even fooled the raikage and his team to think he is Kisame.

There are other problems with this theory to be honest (the fact that Edo-Madara flaty said he brother died for one) but I don't think this is one of them mainly because I file this with the reason Tobi hasn't taken Naruto when he has had countless occasion to do so : just 'cause.
And if you wish for real rationalization let's just say that he has no need for it because he already possesses the power Obito's eyes hold (since let's be honest at this point it's 99% likely that Tobi has -at least- Obito eye)

That's not really what Madara is saying, it's part of Madara's outlook on life that the sole and only thing that matter is power. He laugh the idea that you can bestow ideals or feeling to future generations. His brother died and because of his sacrifice he gained power, as far as Madara is concerned that is the whole of his legacy because power is all there is.
I'll agree it's not a great hint that Izuna is alive (quite the opposite really) but it's also a problem with the idea that Tobi is or has Madara's personality because the two have entirely different outlook on those events (though again, Tobi could have been lying his ass off simply to play on Sasuke's brother complex).