Hi Manimal,
You may be mistaking me for someone else but I have no personal experience of any type with any version of their covers and my own mattress doesn’t have a stretch knit cover (see here).
Your post included a great deal of information but I’m not quite clear on what you are asking me to comment on. If you could be a little more specific or ask some more specific questions it would be very helpful and I’d be happy to reply to the best of my ability 
Phoenix