I would like my wordpress posts be editable by all customers, visitors incorporated. does anybody know if this sounds like possible? I already attempted all of the "wiki" plug ins but with no success.

it's absolutely possible. However, you will find certain step that need manual programming. The step is:

  1. create an edit page in non admin area
  2. perform the editing there (you are able to mimic admin's edit page or else you just display your body only)
  3. upon saving, you are able to, either create new user programmatically and assign that user because the editor (this, off beam will need another table or any other mechanism) or leave the editor as post's previous owner