All interesting points. Personally, I'm still on the Yukio's flame was absorbed by Rin when they were still in the womb. Like: Yukio was too weak so the flame -rejected- him (as Shiro put it) so, where did HIS flame go to? Plus, the moment the first BEI happened where Rin could telepathically "hear" Yukio for a sec (hinting to a connection), plus all the times Yukio's eyes activate is when the Koma sword is sheathed or Rin's not using his flame. Yukio's apparently unconscious will for Rin to "give" him his flame (and I know there're different translations of that line).
But, this whole thing it's still a mystery. Sure, as someone said in the past, it'd be kinda tragic if one day Yukio would find out that the source of all his suffering (his painful desire to be stronger, his inferiority complex toward Rin) was originated by the fact Rin's been "keeping" his (Yukio's) share of power inside of him since birth. And it'd be too tragic for Rin too. As he doesn't have enough to feel guilty about already. So, I hope this is not the case.
I agree Yukio could have some twin connection to Rin because of that moment. You're right that the big question is 'where did the flames go then'? I don't think they are in an independent place in Gehenna with some object like Koumaken cutting him off from it (like Rin's seal). I think they are either back with Satan or with Rin's heart. But isn't there more evidence pointing to the 'with Satan' theory after this chapter? We now know it's been Satan saving Yukio all this time by controlling flames through his eye(s) and that Yukio's been hearing his voice. Aside from the time Yukio actively 'demon-called' out to Rin in fear that time, we don't see any panels of Rin reacting to Yukio using the flames. If Yukio is using from the same source as Rin, wouldn't Rin 'feel' something?
My theory about this is simple. Demon's possession cause severe mutation in the host's gene, although it just seemed magical before the gene and DNA is discovered. And the demon heart ... probably some part of the host body becomes the demon heart after possession ?
That's roughly my alternative view on the whole thing. It still amounts to the same mechanics I think.
Except for the matter of the demon heart. I think they are all born in Gehenna, because it was said Rin's demon heart is in Gehenna, while Kurikara served as window into Gehenna or something...yet it still links to Rin even with the Koumaken serving as a go-between. That's why I think the 'link' established between the Assiah host and the Gehenna demon heart is an important key in creating this kind of offspring. A 'demon' is essentially a being from Gehenna. The traditional/original ones are born from concepts. But when they travel to Assiah they can create demon hearts a different way. And the mechanics of how their children's demon hearts are born/come into existence are unclear. Though I'm thinking a sort of budding off of the parent demon and becoming it's own individual heart is a possibility. In fact it's the only explanation I can think of. I would like to see other possible explanations though.
Come to think of it we have yet to see what would happen if two demons possessing Assiah bodies try to reproduce. I wonder what that would be like.
But why would Shiro had the mood to tell an unnecessary lie to Rin in such urgent situation?
Yeah any theory behind Yukio getting some Satan power doesn't really explain Shiro's actions. Like if he knew Yukio was going to end up like Rin, why give only Rin the special 'sheltered' treatment. I know Yukio could see demons while Rin didn't, but it's hard to reconcile his tough-love behavior toward Yukio without him appearing like a dick if he knew Satan-related sh** would happen to Yukio down the road too.
Maybe Yukio won't have as strong or dangerous powers as Rin or something? I dunno, because even so, Yukio still ended up in a cell at risk of being executed too. It doesn't seem fair.
There's the idea that maybe Shiro didn't know and thought Yukio was normal, but we see flashbacks of him still keeping stuff from Yukio as if he's completely in the
know.
I wonder. Sealing two demon hearts in whatever different methods seems like a boring overkill to me. I still think it's possible that Lucifer's "help you learn how to use your power" and "the secret of your birth" are two unrelated things.
I like the thought that he really didn't quite have demon heart 'flames' ready to connect to him because of his 'weak body' or something, but that they still did something to Yukio as an extra precaution in case it comes back. That would make Shiro look a little less like an asshole.
Or maybe you're right and they are completely unrelated. But if so, why mention it? Yukio knew many things already about it. Mainly that Rin's heart was sealed at the time, Yukio stayed human, and they were raised secretly. Why even bring up 'the secret of their birth' as if there is something still important to learn?