I published 5 novels (4 under one pen name and 1 under another) and am almost ready to publish my 6th.
I wanted to traditionally publish my first novel. I wanted someone in the industry to tell me my novel was publishable. To get that validation (at the time I was only selling short stories to e-zines). The publisher I chose was the largest Romantica (erotic romance) publisher at the time. Their editor was ready to accept my novel if I made a change. The heroine commits adultery (that, IMO, was justified) and that’s a no-no in the romance genre. The editor asked me to change the wife to a fiancee (I guess they don’t consider it adultery if she’s not married). When I explained why she had to be a wife the editor agreed but said her hands were tied so I self-published it. At that point I realized my novels were pushing many boundaries so I continued self-publishing.
When I wrote a mainstream novel, I decided to try the traditional route again. I was interested only in the Big-5 publishers so went looking for an agent. And I queried the really good agencies (one represented authors like Hemingway and Fitzgerald). I only sent out about 5 queries. I quickly got fed up with the process and realized traditional wasn’t for me so I self-published. Being a significantly different genre than my other novels, I published it under a different name.
The mainstream novel was triggered by wattpad. I entered a short story in a wattpad contest and it received an almost perfect score from the judges. But it was the comments from readers who begged for more that made me expand it to a full-length novel. That leads to your #5 question. No, wattpad hasn’t helped me. The people begging for more didn’t buy the novel.
As to your #4 question, first learn how to write fiction. It’s different than other forms of writing. Learn grammar and especially punctuation. And then read. Analyze the story from a reader’s perspective. What did you like? What didn’t you? And analyze it from an author’s perspective (which, to be honest, will take some of the reading enjoyment out of it). How was the author successful in creating characters, suspense, etc.? What kept you turning pages? What would you have done differently? And get feedback on your writing. Grow thick skin when you hear it. And realize that all the feedback isn’t correct (just like, btw, all the articles on how to write fiction aren’t correct).
As to #6, the most difficult part of self-publishing is the marketing. You might have published the best book ever, but if no one knows it’s there it will languish in the sea of invisibility. If your goal is to sell, you better have a marketing strategy that works.