My 6s had a second piece of tape on the opposite side as well. I had a really hard time getting both of these off intact, then realized that if you are replacing the glass/digitizer, these piece of tape are not reused. So you could just cut them to leave an overhanging tab from the EMI shield, then peel them off the EMI shield after it’s been removed.
I think that rubber gasket is what you are trying to peel the microphone away from. In my phone, the gasket is part of the earpiece grill. My screen replacement included a new replacement earpiece grill with a new gasket on it, and new adhesive, so I did want to peel the old one off as shown here.
I hooked the display over the side of my table to support these 6 screw tabs from bellow and they were fairly easy to remove from my late in the run 6s (maybe the factory quit torquing them so hard?) Also, I had a small washer under the last screw 1.5 cm from the edge. Almost lost it since I wasn’t watching for it.