Yes indeed, why do you have to Sign in first! you should be able to do it without that too or is there a certain logic behind this approach?

"Allow comments in text" actually works pretty ok for the purpose I was looking for. The e-mail address is conveyed as a Customer in the backend, this is obviously how it's designed. Another option would have been to generate a Lead from the comment . Appreciate your help!

If we take the odoo business hacks posts as an example it seems that comment in text only works intermittently - some paragaphs you can comment on by hovering, some you cannot. This is evidently not the intended behaviour so must be a bug.