Hmm...I don't know what's wrong with the CrossDOS setup, but you did seem to miss out one file in your description: the actual mount entry. This is either part of DEVS:Mountlist or in DEVS:DOSDrivers/ and that's what you mount. Just a shot in the dark, I guess you know this or you'd never got as far as an error message.
Anyway, if you have access to all the Fish disks, try to get MessyDisk. It'll read MS-DOS formatted floppies as well, on any version of AmigaOS - even 1.3. Worth a try.
As for your rom switcher, there's a very easy way to test it: hold down both mouse buttons as your machine boots into 2.05. If you can get into the boot menu, then there's nothing wrong with your switcher and its the 2.05 install.
