@Kaarme Regarding narrative convenience, I don't see it as any less convenient than when another lesbian romantic love interest appeared, or the frequent "secondary yuri couples" that other yuri manga have. Granted its unlikely in real life, but in real life most gay people aren't usually meeting other gay people through random interactions the way it works in manga. It definitely happens, but its way more common for people to meet via things like dating apps, gay bars, and LGBT events. Most people aren't gay, so bumping into your soul mate at the grocery store or at your job or whatever is just far less likely if you're gay. My point is that yuri manga always stretch the truth on this sort of thing if it leads to a good story. I think this is a good thing to do, as it leads to a wider variety of stories, though admittedly a lot of yuri manga just use it as an excuse for yet another high school yuri manga.
Regarding demographics of yuri - ultimately my comment about a shounen ai relationship was because I think it would have been an interesting reversal of the original premise - instead of "brother helps his sister start a gay relationship," its "sister helps her brother start a gay relationship." I definitely don't think it should have been the focus, but I think you could have had them as a mild secondary couple to mix in with the primary story about the two yuri leads. So more an choice to help pace the plot and flesh out a character than a business-focused demographic one. That being said, I'm not sure I agree regarding the notion that the demographics of a yuri manga would reject such a decision.
If we examine Verena Maser's 2011 study on yuri demographics, we find the breakdown is something like this: 30% were non-heterosexual women, 15.2% were heterosexual women, 4.7% were non-heterosexual men, 39.5% were heterosexual men and 1.2% identified as "other". I think the missing 10% there is people who didn't respond on that question. Note non-heterosexual mostly means a combination of homosexual and bisexual, with a bit of asexuality also. Obviously different manga are going to bias more towards one demographic or another, but I think that gives a good baseline for how yuri demographics work. In contrast to something like lesbian porn, yuri manga is not overwhelmingly consumed by straight men. Instead readers are a mix of many different genders and sexualities, with the overall gender ratio being near 50/50 but biasing a bit toward women.
So given those are the demographics, how can we gauge the reaction to a mild shounen ai relationship? Well, I think its fair to say straight women and non-straight men would generally support a shounen ai side couple. Straight men I think its fair to say would range from neutral to hostile. I think the reaction would be far more neutral than angry for a mild substory without sexualization, and I doubt most of them would drop the manga over this, but I concede that they wouldn't be in favor. The tricky part is in figuring out how non-straight women might respond. Most gay women I've known would be supportive of a shounen ai subplot as long as it wasn't heavily sexualized, and wasn't the focus. This is because while lesbians aren't interested in such a relationship on a sexual level, there is a lot of overlap in the issues gay/lesbian relationships deal with, and thus they can still relate in a general LGBT sense. This is also why I like reading a wide variety of LGBT manga, including shounen ai, as a non-straight woman. I realize this is very anecdotal so your mileage may vary on how much you buy this particular argument, but I think its fair to say non-straight women would at least be neutral, and probably trending to positive when you consider bisexual women, who would probably be on average more supportive than lesbians.
If we sum all of this up, we get a spread of neutral to positive on the female side, and a spread of neutral to negative on the male side. And we've established the gender dynamics of yuri in general are near 50/50. As such I think the overall demographic impact would be neutral, and thus the audience response would be mostly based on execution. If the plotline was clunkily written or overwhelmed the story, I think people would react very poorly. If the plotline was done subtly and didn't take up too many pages, I think it could work as a good way to break up the drama of the main plot with the lesbian love triangle.
tl;dr I think the demographics question is more a mixed bag than a straight up negative, and the success of such a subplot would be based on the execution. But pretty much all stories live or die on their execution far more than the particular tropes they use, so I don't think this is a reason to avoid the troupe entirely. Sorry I went full statistics nerd here, and I don't blame you if you just see a wall of text and say "nope, not reading," but I find it really interesting how the demographics of LGBT manga shape the way the stories are told.
