I dunno. I guess what I'd try first is

(a) grip the wing nut with a large pair of Vise Grips, and then rotate the Vice Grips, OR

(b) find a hex socket that will fit over, but engage, the wing nuts, and then use a ratchet handle.

(c) use a new, sharp 1/8" or 3/16" split-point drill bit, and drill holes all around the through-bolt. Eventually, you'll either relieve the compression on the plug, or cut so much out that you can extract the bolt. Obviously, the pool has to be partially drained with this approach.

. . . membership updated.