Anyone can join whenever. I think the latest we've had someone join was like three weeks before the end.
Depends entirely on the VN. What you're thinking of are called Kinetic VNs where they don't have any choices and you're just clicking through and reading it with visuals and music (and maybe voice acting).
Steins;Gate is not that. I haven't seen the anime but I would assume it largely just follows the True Ending path of the game but the game has five other endings. At least three of them would be considered Bad Endings. I don't know what the general consensus on the other endings. I'd consider two to be Normal Endings. Usually in a VN with choices they come up as buttons to hit on the screen like a choose your own adventure. Sometimes it's picking a direction (go left or go right) but most often it's picking dialogue options. Steins;Gate is more like the latter where all decisions are made by pulling up the lead's phone and replying to texts by selecting from a few options. Or choices can be made by inaction of not sending a text or answering a phone call. There's actually a lot of branching paths with some having zero outcome and just for flavor, many leading to CGs (fancier full screen scenes), and achievements. But they also are what lead to the alternate endings.
No real spoiler to take a quick glance but you can click this link and scroll down to see how all the chapters have a bunch of branches:
Steins;Gate 100% Completion Walkthrough