My apologies for the delayed response, but here is more suggestive stuff.

My original comments were more about fans than canon.  Some fans are explicit about it.  “I hate Cho, but I don’t know why.”  “I hate Ron because he won’t admit his feelings for Hermione.”  You extended the suggestions to include more of canon.

Considering canon, the first threat that I can think of was Lockhart who is soon revealed as unsuitable.  The most potent threat may have been Sirius who is dealt with harshly: He is not allowed to mature into an appropriate role model.  Arthur and Molly Weasley strike me as having made positive contributions to the cohesion of the Trio.

In terms of loose ends, if I’ve heard correctly, Lavender does not survive in the movie although she does in text.  I attributed that to Hollywood where if there is a romantic triangle, one of them must die.  Unrequited love is forbidden.  In contrast, unrequited love is a major driver in the books although Severus’s life may have been a living death.  (I feel slow since it was halfway through the second volume before I realized Severus was not going to survive.  My question was how redemptive would his death be.  Reports are “not very.”)

I always interpreted faithful and gentle Ron as a single mother’s idealized companion.  This appeals to some while others believe he is lacking in all other departments.  I regard Hermione-Severus as desperation since the wizard equivalent of Hermione would be more than the Trio could withstand and true fans do not want to write an original character.  (A popular theory is that fans are reinterpreting Severus as Darcy.)

Thinking about fans, myself included, I have quite mixed opinions and feelings. I must admit that when I discovered HP fanfiction, I was dumbfounded by the massive Hermione-Severus as the only OTP, quite vehemently insisted on, and any other proposed pairing ridiculed and dismissed... also that some fans will sometimes, rarely, give a chance to Severus and an original character, but Hermione with an original character seems to push their patience, which is ridiculous, imho. Fans are reinterpreting, plagiarizing, Severus as Darcy--lol! Good Lord, so that's where it's all coming from--well, it is fan fiction, and Darcy is well known, easier to slot him in and give him a make over via Severus, whereas an original character would take some creative, organic effort, especially to be up to par with Hermione... Although it has always come across to me that, yes, Severus' life was a living hell, a miserable exsistence, a continued torturous unrequited love marring and dictating his thoughts and actions. Perhaps, for many fans, the inability to accept this vital aspect of his character make up (for JKR was quite brutally merciless regarding Severus, imho) had led them to flatter and play with imitating Austen, this being considered in hive mind manifestations--such as fandoms inherently are--being more properly pleasant and acceptable by society mores, etc., although I'd go with haunted Colonel Brandon rather than the tedious Darcy--hehe! Sorry to be so flippant, I'm a Brontë nerd girl any day of the week rather than an Austen one.Yes, I read somewhere or heard through the grapevine that Ron was indeed based on Rowling's childhood friend who she had unrequited love for (she'd wished that they had become more; it never happened; he had the great car that they would escape in from her miserable family home life, etc.), so he is a single mother's idealized companion: faithul, gentle, will never leave your side through thick and thin...I think I also heard somewhere that Rowling is a huge Austen fan, herself being heavily influenced by Austen and Dickens, so fans imitate her/their styles...On an objective note, well, maybe not so objective on my part as I work half of the time in the education field, I suppose it's a good thing on one level that fan fiction causes readers and authors of all backgrounds to be interactive and involved in the world of literature, but again, I'm always surprised that so many have very specific criteria, limited, on what they will be open to... but that is their right--to choose, the freedom of choice and selectivity... Hmmm... Your presenting this topic about fans' strong reactions to Cho, Ron, Lavender, etc., has made me reflect a lot about things, as 'hate' is a very strong word, and fans use it left and right in ultimative comments about a character and then dismiss the ones repellent to them as if they are not part of the world of the canon and definitely never ever exploring them further in their writing (although we are given only a handful of named characters as the Wizarding world consists of (unkown) thousands--I always ask myself 'What about the others? what are their names, backgrounds, stories, etc.--the possibilities could be endless.)--much safer and acceptable to the majority of others to try to write the Hermione & Darcy formulae, I suppose... as well as how each reader/writer, I suppose, has categorised each character and labelled them in canon as they came across her/him. It's too easy to say it's all subjective, and to each his own--as you have brought up some very stimulating and interesting observations! I want to think some more about all of the threats to the Trio. Again, thanks for presenting this/these topic(s)! Food for thought!