I was also surprised about the mouse when I got my A1000 (got mine for free). An external floppy will definitely come in handy (got an external floppy for free too! ha!).
Unfortunately, you won't be able to put the kickstart in one drive and WB in the other. Only the experimental kickstart 1.4 could boot WB from external - and 1.4 is a little too buggy to be useful.
About removing the case - after removing the case screws, there are snaps on the inside of both sides of the case. You just have to make sure you push in - I think on the bottom part (going from memory here) and carefully pull out and up on the top part to clear the snap. Then do the same on the other side. It's a bit of a bugger, but go slow and careful so you don't break it.