Most of these questions can be answered with it depends! It’s really up to the writer and what works best for them, but let me try to be more specific.
Some writers keep their WIPs up from beginning to submission (I believe KOTSAM did this); others stop updating the public demo at a cut off point (Mind Blind’s public demo ends at chapter 9).
After a game is published, the only demo available is the normal (usually first 3 chapters, I think) demo that COG/HG publishes. Earlier demos are removed from the WIP thread/dashingdon/etc
To be published, you have to do a beta. Some authors collect beta testers through the forum, others have their own groups that beta their work–or a combination. Some authors do beta signups or applications. It’s fairly typical, I think, to start with a public beta (or alpha) and move to a private beta after getting to a certain point.
I don’t think there is a “typical.” A lot of writers, however, are active both in their WIP threads and on tumblr. Discord is another popular medium, but I’m not sure about other platforms. AFAIK, twitter and facebook aren’t common, but I think there’s a growing reddit community now.
They commission artists or do it themselves! You can find artists offering their skills in the professional services heading, and you can also take a look at cover art for previous games and see if there’s an artist/style you like and consider commissioning that artist to do some work for you.
Here’s a guide to help you make chapter headings! I’m no good at it myself, so I wish you better luck
This one is harder to answer more specifically than it depends. Longer demos are going to be better for generating feedback that can help you. It is difficult to give feedback on only a few hundred words, for example. On the other hand, you probably don’t want to wait so long that making changes to earlier chapters (which will also affect later chapters) becomes difficult. The purpose of posting a demo/creating a WIP thread is to prepare your game for publication–this might mean different things to different people! Some people might be looking for bug reports, copy editing, etc while others might be more interested in high and mid level feedback. In my experience, most writers are interested in all three (and that also includes positive feedback and encouragement!). The important thing for readers is to pay attention to what kind of feedback writers are looking for, and on the flip side the important thing for writers is to communicate what kind of feedback they want to hear.
I hope that’s helpful to you, and although I’m no expert, feel free to send me a PM if you want to chat more about something 