Incredible. A doujin settled between Chapter 4 Part 1 and Part 2 of the main story that is not afraid to address the hard and spoiler heavy topics that make Heaven Burns Red so fascinating and philosophical, while still retaining the humorous and absurd moments that make it endearing.
Indeed if Ruka needs to etch a strong memory of Yukki into her soul so she can never forget her... this would certainly be one way. At the beginning I was immediately reminded of Ichiko Oshima's event story which also revolved around memory loss, but the author had already thought of that and brought it up directly. Fortunately it did not take such a dramatic and dangerous trigger to recover her memories as it did in that event.
I have no doubt in my heart that Ruka and Yukki will eventually reach this point naturally in the canon. Memory loss aside, this felt completely in character already. As expected of the main couple I suppose.