1) A design experience (DE) is something that the studio offered with specific packages (ie expensive) dating back to the kickstarter campaign. They are no longer available, but if you look up the CoE KS campaign (it's still accessible), you'll see some of the things they offered. Examples include custom fighting styles, weapon designs, armor designs, animal designs, hairstyles, tattoos, food/drink/potion.
2) The process for creating these will hopefully be clarified in time - for now, we have not been informed how they will be created - but DE's cannot be purchased anymore.
4) Yes. They were only available as part of specific packages, typically the most expensive ones, like the nobility packages.
5) These were defined by the packages and they varied by package. See #1 for examples.