How to remove trackpad mounting flanges?

I asked a question a couple years ago about a Stuck tri-point trackpad screw on a trackpad in a 2008 Aluminum MacBook. At the time I couldn't figure out how to get it out (problem wasn't really solved, despite the heading on the question page), so I set the thing aside. I just tried again, and by jamming it with several different screw drivers I ended up detaching the head of the screw, so the trackpad could be removed. (The body of the screw is still stuck in the trackpad, and I suppose would require some sort of micro-drill to get it out, if at all. However, as the other five screw holes are clear and the trackpad itself is good, it can still be used, with only five screws holding it in place.)

One of the flanges which these tiny Tri-point screws go through to attach the trackpad to them is somewhat messed up, so I'd like to remove them and replace them with the two new ones that came with the new trackpad I got (before I knew that the existing one was still usable). Unfortunately, the Trackpad Replacement Guide has no information about how to do this, simply assuming that the flanges remain like new (as in the photos in the Guide) and can be reused.

The flanges appear to slip into slots in the upper case, and look to be held in place by screws (2 each) sunk into the upper case above them, but I can't tell what sort of driver would remove them. They look to have a six-pointed form, like torx screws, but with a conical cavity beneath – while the torx drivers I have have flat tips rather than the pointed tips found on phillips or tri-point drivers. The T-6 driver seems to sort-of fit the screws, but I'm reluctant to try anything for fear of stripping them.

Does anybody know exactly what kind of driver is needed to remove these screws? Or any ideas about how to get them out, so the flanges can be replaced?