My experience knocking dents out of the 850 fender has been mixed. As Klaus said, metal on metal is a definite no-no.

The only trouble I have had is the contour of the fender is hard to recover if it has been affected. For example, after correcting a large dent my driver's fender and fender liner were scraping the outside of my tire. Try as I might I couldn't modify it enough to be correct. I finally gave up (decided a new fender was needed) and just grabbed it and yanked up. Not the best body work.

To answer one of your questions, in my experience making corrections with the fender on the car, depending on what is needed, is very difficult. You don't have any room to swing your hammer!

I must have been very lucky - I found a painted (my color) used passenger fender in good shape on ebay for cheap. Sadly, that is now my good fender, since the driver's side was hit shortly thereafter...