1. Massively depends on your personal pace. A solo player can certainly do their first run in under 100 hours without assistance, but some might take much longer on things like building or overstocking. Knowing the game well though, much faster, not even a fifth of that time.

2. Not really. If anything will make the game more replayable it is the people that you play with. There isn't really a way to play successive runs radically differently without purposely adding tedium like "don't use X". Cheap, unimaginative, tedious stuff. But you can always see what mods have to offer.
One world is enough to see it all because despite the random generation, the difference between two places using the same biome are largely superficial.
Of course stuff like building is what you make of it. It could add hundreds of hours more for someone who really likes its implementation, but it isn't necessarily going to exceed your imagination any more than other games featuring building mechanics.

3. Yesn't.

4. Planned is not the right word. They know it will happen eventually. Earlier last year they hamfistedly announced a lot of stuff when they had nothing more than their names. A lot of it still may not be started. At the current pace the 1.0 build will certainly not be 2022. Possibly 2023 if they do pick it up like they said they would.

I do however think it is a good price, even for the content that we already have. Less cost than most other games in the genre, but also for less to see and do. It is a promise-powered investment.