Hi I just had the same problem with the batteries and the covers stuck. I sprayed the top of the cap with WD40 and left it for about one hour. Came back, grabbed a big long screw driver and opened it fairly easy.
Now I have to figure out to get the batteries out without destroying the keyboard.
I sprayed the batteries with the WD40 but no luck, I fumbled with a knife, nothing... all three batteries are stuck, hopefully its just the first one.