it's a shame, but I guess a novice writer is bound to end up relying on cheap drama when they run out of ideas.
I thought the same when Akira was introduced, and look how wrong I was. Instead of a standard love triangle, we got a character who actually served quite an important role in the progression of the main relationships, by bringing up some really pertinent points, like the fact their relationship could turn sour, as many romantic relationships do, and Asuka is still a teenager, dependant on her mother for basic life needs. True, Asuka still maintains her resolve to be with her mother now, rather than later, but at the very least she is now aware of the issue, as evident when she straight up tells Jun here she is not the mature one.
Akira also made Ayako think about Asuka's father, which was a good thing because in a relationship such as this one it was bound to come up sooner or later (evident by the fact people in this very thread have dreaded the substitute-for-a-deceased-parent trope since the very beginning), and it is a question that needs to be dealt with, regardless of the final outcome.
Not to mention Akira managed to intrigue me as a character of her own, and I would not actually be opposed to her returning. Considering I went into a full blown love triangle alert when she was introduced, and was quite harsh on the author for it, I think it is a pretty safe bet the author actually knows what they are doing.
As for Jun, as people have already pointed out, Asuka is not interested in him, he knows she is not interested in him, he has no intention of pursuing her, so quite frankly, I just do not see the het angle here. My guess is he will serve as a means for Asuka to reflect on how society at large might view her incestuous relationship.
I am actually kind of on the fence about how he will react, because Akira turned out to be pretty relaxed with the whole mother-daughter issue (in her own family, no less), I somehow doubt the author will go for it the second time. Some people are open minded and non-judgemental when it comes to incest, but let us face it, the majority of people are not like that. In a mango that decided to deal with the whole relationship in something of a believable fashion, I am reasonably certain some hostility against the main couple is bound to happen, and the impact would be greater if it comes from an old friend, rather than some random strangers.