Question

For two ideal polarizers, the light intensity when the polarizers are at 90 should be zero. Why was the light intensity for the two polarizers at 90 not zero?

Answer 1

The light has no memory. When it passes through the second polariser, there is no information whatsoever of its previous polarisation. It could have been whatever!

When light goes through a polariser at 90ºand 90º, it gets tilted at the cost of some lost intensity. In other words, it gets projected on the polariser axis, that is rotated a bit with respect to the light polarisation vector. This new light then goes on to the next one, and gets tilted again.

You get the picture. You can use this idea to arbitrarily change the polarisation angle of light by putting many of them in series. The more steps you do, the more polarisers you use, and thus, the smaller the steps, more light will come out on the other side (in reality, you always loose some light because they are not completely transparent, so you don't want to use many of them).