How do I install a window screen that must go in from the outside and does not have tabs, springs, pins, or any flex?
They don't all use springs or clips. On many (especially older aluminum and steel frame) windows the channel is deeper on the top and one side. This allows the screen frame to insert, then drop and/or be pulled into the shallower channel whilst still being (half) held in the deeper channel opposite. Usually this type has a plastic "pull-tab" that is held in by the rubber spline, to help the user insert and remove it. It is common to see these tabs break off over time.

None are made to have the screen frame bent for insertion.