Bottom screw was totally stuck, nothing could budge it. I removed top screw, heated up battery with hair dryer to release the glue, pulled it out using the spudger. Lifted up the metal plate thing (after taking out the ground piece and putting it aside). Cut off the flexible bit between the plate and the battery and twisted the plate around the screw as you did.
Well, the screw eventually came out—but only the top half! Yes, it split in two. Then the bottom half came out, still attached to the thing it was screwed in to (a sort of barrel).
I put the new battery in, put the bottom half (with the barrel) of lower screw back in its original position, replaced the ground piece and positioned the new plate over it, and screwed the top screw back in. Then put the top half of the lower screw in and tightened it.
Extra: It was helpful to discover that the 1.5 and 1.7 mm measurements refer to length of screw, not size of Phillips head.