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Regrettably, I'm afraid I can't be of further help if the screws are unable to be removed. I will opt out to allow another expert to jump in if they have further ideas, but it may be more beneficial for you to contact Toshiba re: service.
Hi there, I have a video guide for you that may be of more use that discusses some of the tools and methods used to do this more efficiently, i think that toshiba uses star bits, rather than all phillips head, in some of them like my Asus laptop does... please have a look here and let me know if it helps
the video is to show how to replace your DC Jack, but it shows how to remove the bottom cover as well.