Kito has a good idea about just adding another another rope stopper.
The other idea about using more PB Blaster over several days, and trying the impact driver is also a good idea. That's why they use slot head not phillip's head bolts, you can use a lot of force on them using that impact driver, without stripping the heads. Good luck, and let us know how this comes out.
If you are able to get it free (when you are able to get it free), bolting the line stopper into the aluminum plate , by tapping new threads, is plenty strong.