In my textbook and wikipedia, I have observed that force exerted on a wall of the container by one molecule is taken in account. Such that $F=\frac{mu} {\Delta t}$ where ${\Delta t}=\frac{2u}{l}$. But this change in time is the time required for a molecule to move from one wall to the opposite. In a gas container, each gas molecules don't get to move this much freely. Then why do we assume ${\Delta t}=\frac{2u}{l}$? Is it that the molecules remain in random motion and tends to maintain constant density all over the place for which the statistical value of $\Delta t$ turns out to be the same?

Another small question, were polyatomic molecules also considered as one sphere each in kinetic theory of gas? Or was it each atom resembled a sphere but not a molecule?