It's just my impression or Tsunderes really tend to be tragic characters?
Well.. Tsundere's originally were intended to be new interesting characters, away from the Yamato Nadeshikos and Plucky Girls of that time and inspired by feminist protesters of the 1970s. Dunno if that makes them tragic. Perhaps times are changing again and their behaviour is reinterpreted.
It's up to the media genre whatever they become tragic characters. If it's drama, then probably yes. If it's comedy, then perhap no, except when True Art Is Angsty and/or Cerebus Syndrome are at work, especially since people these day tends to love tragic and deconstruction tropes.
Strange it hasn't been said before, but this behavior sounds very much like bipolar.
What? Bipolar switches between depressed and hyperactive for extended periods of time, usually weeks or even months. Tsunderes are angry or nice, and can switch in seconds. The only "bipolar" you're seeing is the fact there's two distinct personality modes.
Is borderline personality disorder a better fit?
No, BPD is bigger than that. It involves self image, black/white thinking, tendency to self-harm and characterized by agression and depression rather than affection. Romantic relationships are just one of the places where it might manifest.
Tsundere is a specifically romance-related trope for characters who can fell anywhere in the general behavior spectrum (A character could be The Messiah and still be tsundere towards their Love Interest). Meanwhile, Jerk with a Heart of Gold deals more with how the character interacts with everyone around them.
A tsundere is a person who usually acts like one of the tsuntsun/deredere personalities, but switches to the other at various times, whereas a Jerk With a Heart of Gold always acts like a dick, but is also a good person deep inside.
This is rather random but my friend and I have started to notice alot of misuse of the term "tsundere."
We've been reading alot of weaboo horror stories and one of the most common themes is that boy/girl will be pursued relentlessly by weaboo boy/girl only to tell them to back off/I don't like you that way/ we aren't friends/ I'm highly uncomfortable with your advances/ I'm gay/ I'm straight/ I strongly dislike you/ I have a significant other/I loathe you, or a multitude of other deterrants. Many even going so far as to try and completely block the interested party from their life but each time are met with a "Hee hee, you're so tsundere it's adorable."
Another possible explanation: the pursuer is confusing tsundere behavior for someone Playing Hard To Get.
Why is it that Tsunderes are the most common harem winners? Is it because they are portrayed the most realistically? I mean, how many girls do we know are generally nice towards practically everyone except the guys they like?
For a harem series, not really. The entire genre is pretty much based on miscommunication, which is the bread and butter of tsundere characters. The author can introduce and "resolve" the runners-up while still keeping up the Will They or Won't They? interest in the Official Couple. The only shows that really do all that well without a Tsundere-type lead are those that push the Fanservice to the limits of good taste.
Has anyone else noticed that Type A Tsunderes often make it difficult to tell the difference between their being "tsun" and when they frankly do not like the character in question, especially considering that most Tsunderes I've seen have regarded the person they're tsundere toward this way at one point or another?
This is because they might start off annoyed by the person they're tsundere to, but eventually start noticing that person's good points.