Hi! Ultimately, I think it will come to what you’re describing (although I don’t think he’s moving within his dragon kakuja at all), especially since we know Touka was last seen following where he possibly was…

so for now I think she’s the one who is going to get him out. However, before we get there, I’m of the opinion that the current cinematic playing in his head is all like a trial he has to overcome, before he will allow the waves to pull his body to “the other side”, through that sea of dead bodies, and he’ll emerge where Touka and everyone else are.

See, since it’s all in his head, it means that on some level he’s already conscious of what he did ever since ch144 (hence why what you described does look like Dragoneki’s eyes) and “Rize” is just supposed to help him reach bigger conclusions, because she was used as a catalyst to major realizations before.

To be honest though, and to address your question of if Dragon is somehow represented in ch157, I wonder if Rize isn’t also representing Dragon on some level. What I mean by that is that Kaneki received her kakuhou but Rize isn’t such a random ghoul, because she’s from the Washuu lineage… 

…which is why Dragoneki currently seems to parallel the Nagaraj from around 100 years ago so much.
So, more than just Rize acting as a catalyst, I wonder if she’s not also supposed to make him see in what kind of net he fell from the very beginning, and what role he is supposed to acknowledge next. 

I mean, if you look at Kaneki’s narrative arc so far, he was stuck in the position of the OEK and was not doing a very good job of it, which is what led to Dragon…

Then again, the whole problematic of the OEK is that it’s a title that Kaneki received from Arima and Eto, a title he didn’t particularly want but that he still took because, at the time, he had nothing else to live for.

Honestly, becoming the OEK should have been easy enough for Kaneki if he had cared about coexistence in the slightest, because the “OEK” terrifies the CCG and V due to the previous OEKs who raised havoc in the past:

However, Kaneki doesn’t care about coexistence, at least not for now…

…which is why he failed to use that title correctly and the OEK will possibly be no more once the Dragoneki arc is over. 

That being said, Kaneki always was presented to be the bridge between humans and ghouls, and that from TG’s very first volume, so he’s not going to be able to escape this role by now. 

Anyway, back to Rize = a possible form of Dragoneki symbolism, because he was given Rize’s Washuu kakuhou, Kaneki was a pawn from the very beginning in Kanou’s/Furuta’s/Eto’s plans to annihilate the Washuus and current system, and I think it’s highly time for him to realize that too, because he has his own freedom and future to fight for as well, until this conflict ends.

That’s why I see the current cinematic in his head as a trial he has to overcome. There is what Rize literally called him on…

…which is a reality he has to acknowledge, because he killed both ghouls and humans by now. However, there is also the fact that his whole narrative of being a OEG + being referred to as the bridge between humans and ghouls who would bring coexistence still exists in the plot, and he can’t avoid this reality either when he once embraced it in TG until Aogiri fucked everything up.

In other words…

I hope that Kaneki will realize once more about “times when you have to protect something even if it means throwing away something else”.

It will start with acknowledging that he became a murderer, long ago for ghouls and recently for humans, for this very reason (which is not particularly an issue at this point since 99.5% of the cast is the same).
However it might also allow him to realize what kind of role he’ll have to tackle next (maybe the Nameless King if there is a real difference with the OEK?), since he won’t be able to have that peaceful future with Touka and their baby as long as this conflict doesn’t end. 

Sorry for rambling. I swear I didn’t want to digress xD To sum up, my thoughts are that Rize is a trigger to realizing a truth he’s already semi-conscious about (hence the whole cinematic, since everything is happening in his head) but never worked towards before. So it’s not just supposed to be about looking back on the past, “Rize” is also here, in my opinion, to make him realize what he’ll have to do in the future too. 

And once he’ll acknowledge all of what she has to show him, once he’ll be done with this new trial that he created for himself, then he’ll be able to swim through that water without fearing those dead bodies and with Touka waiting for him on the other end. 

I hope it was understandable, sorry it got so long. Have a nice day!