I once bought a couch that had attached cushions, that were pretty grimey looking. I got the Resolve "dry" powder upholstery cleaner that comes with a brush, and used it to remove all the grime. I did have to do a couple spots twice, but it looked new when finished. Might work as well for you. It does work on microfiber.

My mum used to have a couch in this fabric and I used to clean it with either water and baby shampoo or use the steam cleaner on it, both worked well as hers was a very light colour.

I have seen some nice heavy duty covers on QVC. They advertized that they are washable and pets don't mess them up.

Here's a thought ... if you plan on re-upholstering the whole sofa anyway ... why not just cut, or rip, the side seams where the cushions attach to the sides? Typically when one re-uphosters furniture, the hold upholstery is removed anyway. Right?