#2 - A Netlify staff member would be able to confirm, but I know from experience that the blog articles aren’t maintained in the same way the documentation is. So you should preference the documentation.
#3 - This is the support guide everyone gets pointed to when they encounter this (and generally all form issues).
#4 - I don’t actually work with Gatsby or Netlify Forms, so can’t say for certain, you could take a look at this thread which mentions the same issue. If that doesn’t help (or searching for other similar threads) I’d imagine Netlify support will be able to point you in the right direction.