There are some cool thoughts here. I'll start from the bottom up, because I feel I have most expertise in your last three questions.
Q.5: Web novels and traditional, paper back novels are different in that web novel authors tend to inflate word count, or draw out arcs, in order to hold readers for as long as possible. My background is in traditional novels, so when I was writing those, my editor hammered me constantly. "Be concise!" he seemed to shout in his emails. "I can reduce your entire novel by 30%!" A 30% reduction on a 600 page novel is crazy to imagine. He thought I wasted too many words, and he was right, frankly.
My method back then was to keep each scene in the area of 2k words, and thereby keep each chapter about 10k words long. Currently, I do it mostly the same. I keep each chapter around 2k words, but if it's 500 words less or more? That's fine. I write only what's absolutely necessary, either for the reader or for myself. While I try to keep within a range, if a scene is only 800 words? Fine. Bloat is distracting, it's complicating, and this reflects on your third question; write too much and the details that matter, like chekhov's gun, will be lost or forgotten.
Q.4: Very rarely does something have to be exact. Only be exact if it makes sense for a smart character or funny narrator. As a general rule, keep things vague in the beginning, and if it's a recurring theme or event, get more precise as you go along. It will help you remember, and it will keep it in your reader's mind. For instance, start with a reference to a battle, then if it comes up again offer more details about what happened; eventually, someone might mention the date, and if it has mattered for so long, it'll be an important plot point you will remember.
Q.3: I always try to include details about characters, even the minor ones, because I want to create a good image in my reader's mind. I'm taking them on an adventure, to my place, filled with strange people and strange things, so they should have a good idea of what they're looking at. Not everyone believes this is important, and depending on your genre and your audience, it might not be, but it's what I like.
But, of course, the characters in my novel actually have special eyes! So, it's often important to talk about eye color. It's also important to talk about age and gender, skin color, or clothing style, because how people look and who they are affects how characters interact. I may not often mention how tall the king is, but I do mention it, and it affects every scene he's in. He can be thoughtful and gentle, but he can overpower people with his size, and that has an impact on the reader's perception of him... and the house nobles, who surround him like children; look to him for guidance.
But, remember, the rule is to say only what's necessary. Be concise, says the harsh editor! If a character's 'character' doesn't influence the scene, perhaps you should rethink them, or include a wider representation of ages, genders, races, professions, etc. Sometimes, a character who pops in just to deliver a letter doesn't need an introduction; sometimes, that character starts a conversation in the room, and then maybe it's worth mentioning that he's a down-and-out noble scion, working as a page while his house is in disarray.
In all things, use your best judgment. Write the way you want to write, and your work will find an audience. If that audience is large enough to earn a living is another conversation.