I stand to be corrected, but when me and a couple of mates got together a few months ago to help remove the same wing from my friend's R34 GTT... after having done what you've done so far, we then had to unbolt 2-4 other bots holding the wing in place from within the boot itself from the inside.
What you'll have to do and I caution you that if you don't do this delicately, there will be a huge tear in the teal inside your boot which makes it ugly as... but yeah, you'll have to cut the sheet of metal from the inside of the boot and unscew the boot that way.
That's what we had to do, not sure if there are any other ways...
Good luck.