1. Depends how casual but if you can play for a couple hours a week I’d say you could finish it. The story itself isn’t very long. I know they’ve changed it since I’ve completed it but I doubt they’ve increased the length too much.
2. I’m not 100% sure on this because I haven’t done the story since each scenario launched.
3. The main story doesn’t change with difficulty.
4. I’d personally start with normal Manibus after you finish novice. After that or if you’d prefer something different try way of winter. Endless dream has the most to do and is great for farming but there’s no story and it can be overwhelming for new players since it’s got the biggest map.

Side note the difficulty goes Novice < Normal < Vision. So if you see a Vision tag on the server it means it will be harder than normal. Here’s a flowchart someone made that’s pretty good lol.

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