I have decided to have "underclassmen unable to declare" as my selected option for my save. So, does this then include 5th year seniors as well? In other words, is a 5th year senior a senior can can declare for the draft in his fourth year?

Since all seniors--whether marked with an (R) or not--are going to graduate, there is no point to this question.

If you mean 'redshirt juniors', they are underclassmen (kind of the point, right?) they cannot declare.