This question gets asked too many times!
Of course, there are examples where this is the case like Shenmue III or Broken Age, but in reality, Kickstarter is a terrible way to ‘market’ your game.
You should only use the platform if you’re genuinely looking for funding for your title. Running a Kickstarter campaign is a lot of work so if you’re looking for an ‘easy marketing hit’ look elsewhere.
If possible, yes.
Offering a demo is a great way to get a community excited about your game. It can offer backers a piece of mind, knowing that what they are backing has made significant progress as a game.
However, don’t just put a demo out for the sake of it. If a demo is available then 90% of backers will play before they back, if the demo feels underdeveloped or offers a poor playing experience you’re going to steer more backers away than what you’re going to gain.