I have had this issue on multiple occasions. Even after trying all of the things you have listed and more I have found a solution which is my go-to in situations like this and has gotten me out of sticky situations multiple times. You need to get the ifixit driver bit in a flathead size 1. I have tried so many other flathead bits without any success and this one seriously is a god sent! Even when it is completely rounded off, I have found this will remove any pentalobe with the right amount of pressure. Can be purchased here: https://australia.ifixit.com/products/ma...-driver-kit-64-precision-bits?_pos=4&_sid=ccac46018&_ss=r Good luck!