The idea behind not learning the laws of mourning, is a superstition that this could cause someone to die, and the laws to become applied (see Hattam Sofer YD 346, and a discussion of the idea here). R. Elyashiv is quoted as rejecting this idea here. Similarly, this article quotes Yossef Omets (pg. 270) as referencing this superstition, but nevertheless encouraging the study of the laws. There is also a famous passage in Sefer Hassidim (291) encouraging the study of Moed Kattan and the third chapter of Berakhot. Presumably this was due to a superstition about studying death related material, but the Sefer Hassidim nevertheless extols such study. The article also infers from Igrot Moshe (YD Vol. IV:60:4) that there is no problem for non-mourners to study the material. R. Hayyim Kanievsky is similarly quoted (Shu"t Hok Heshiv YD: 5) as stating not to be concerned with this superstition.
The idea behind not learning the laws of Nidda is presumably that it is inappropriate / and or may arouse inappropriate thoughts. However, a Rosh Yeshiva I know felt there was no problem. (Presumably if an individual felt he had a problem learning the material, then that individual should refrain, but he felt there was no universal default problem.)