slowtostart: Do you have a close-up or a different angle shot of that rack? It looks like it's a weird shape. The axle bolt and the fender eyelets are very different sizes. I'm not sure how you would switch from one to the other unless there are different sized holes on the rack supports. However, if it's possible I would get it mounted to the eyelets. Taking the back wheel off for flats etc. will be less hassle if you don't have to deal with the rack also.
I've never kept old chains because the ones I come across are always really gummed up. They may be in decent shape under all that gunk but chains are cheap enough that I always just put a new one on.