There are two fires. At least I
think the first one where Riku dies was another fire judging by the background illustrations during that part.
About Hanyuu being alive...
these details I'm unsure about as well:
She may have died too (so... she died twice...? Did she regain a physical form again to be killed by Ouka later on? Someone needs to clarify this again for me...), because it seems like
no one else can see her, much like how Rika can only see Hanyuu during Showa 58. When she first meets Ouka when she's ten-years-old, Hanyuu is surprised and asks... "You can see me?"
Oh, and... little Ouka also uses "nipa~" and "mii~" much like Rika does. (Little Ouka goes "mii~" the first time she sees Hanyuu)
The arc ends a little after older Ouka kills Hanyuu (Hanyuu even has a crazy laugh... ;; and it appears this is the only time in the arc where Ouka finally calls her "okaasan") during the second fire. I can't read it, but during the credits, it looks like it may be a description of the aftermath.
I think this is the most I can tell you about this arc.