1.) Do short chapters turn you away from the reading experience?
No, and the fact that this is a thing makes me actually pretty annoyed. Like if it's a free webcomic, you get what you get.
2.) Would you not read a webtoon because of its art style?
Different art styles have different visual languages to them. So, if you aren't used to manga, then you're going to have a hard time reading it, because they have different unspoken visual tells to them. Same with Western comics, same with indie, and I think even the Webtoon anime style is a little different from page format manga style enough that if you aren't used to it--you're just not going to have as good of an experience.
It doesn't mean you can't learn, but it does mean that there's a period of time going into any comic (especially comics that have a very unique styles) where you have to learn that unique visual language, and that can be a turn off for certain people who just aren't here to learn a language. Me, I like diverse styles, but I understand if it's not everyone's cup of tea.
So I don't want to say it has to be "good" because I'll totally read MS paint stick figure comics if that is the style that works best for that visual language of gag humor--but it has to be a style that works with the story being told.
3.) Can good art fix a bad plot/story?
Never!
I have learned from experience that you can't put make-up on a pig!
4.) Can a good story make you stay reading even if the art is not your taste?
Yeah? Kind of? So comics are a medium where art and story are intertwined. The storytelling is half done through art, so if your art is lacking, there goes half your story. I can't think of any instance where I was like "so glad the story is good because this art succcks" Like, I just can't.
But again, I think "good art" is subjective. Some people think good art has to look like a movie frame or a photoreal situation, but I think that's sort of not the point of a comic. When I think of comic art, I think of like...TinTin, which was simplified so that they could meet the demand of how much they needed to write and how quickly they had to write it. So for me art is a stylistic choice, and style is something that comes out of restriction of time and money, so I don't judge if they don't have time for shadows or making perfectly on model characters. Lets just say I live in a glass house in that regard. 
5.) Is there anything that is a complete turn-off when reading the first few chapters of a webtoon?
For me the biggest sin is if it's freakin boring. Make something happen. Blow something up. Something needs to be happening and interesting in the very first episode.