Note how careful @A.T. was in his phrasing here. The molecules are not static and not arranged in columns. But for Pascal's law to apply, the fluid is static. An analysis can consider a columnar arrangement since the net forces external to a column of fluid are balanced and are exactly right to keep the fluid within the column (or horizontal tube) stationary.The molecules in a fluid are neither static nor arranged in vertical columns.
Yes.It must be equal to the pressure on the top surface so that the fluid doesn't flow?
It must be equal so that it doesn't flow.Now consider a horizontal tube running from the side of that cube over to another cube in a portion of the fluid that is hiding under a "ceiling". Is the pressure on the right end of that tube equal to the pressure at the left end?
Yes, that is it. That is how you arrive at Pascal's principle.It must be equal so that it doesn't flow.
Then for that cubical part to not flow the pressure on top surface must be equal to that of the pressure on the side surface. Hence, pressure on both the cubical part is equal. Is it?