1. Too easy
.
2. option might reward you with some "sharp" edges while pressing the buttons, besides this, I would prefer it. It allows the easy travel down and not cause addition friction compared to the first option (which might have the sides sliding against each other if some wrapping occurs).
3. & 4. might cause the button to jam if shifted to the side, so it wont release instant - but this won't be a problem if you make both thicker around the cut. Also might be heavy to clean if needed.
5. You'll be unable to clean the cut, you cannot slip some paper in there to catch the dirt - or you have to lift the button, dunno if the material allows that much flex.
U or O? Going for the O also asks for an smooth and low frictional guiding of the "keycap". The U should be easier since it's attached to the mouse itself, only needing a thinner part before the switch starts - but this will also give you the possibility to add some extra force for pressing the switch.