1. First of all, there are many ways to draw straight lines on different planes. I think the best way is to draw two parallel planes first. As you said, the upper and lower sides of the rectangle are parallel planes, then draw a straight line A on one plane, and then draw a straight line B on the other plane. Then a and b are straight lines in different planes.

2. What you said makes sense. For example, a closed figure with four equilateral sides on a plane is a parallelogram, but this is not practical in space, and you said that some corners may be narrowed, which is only visual, just like if you look at a cube, some corners look 45 degrees.

One more thing, in fact, some angles seem to have increased. Let's take a cube as an example Some look like 145 degrees, but they are actually 90 degrees.

3. In the end, they will not actually decrease.