Is it true that you aren’t supposed to visit graves on Saturday? And why? If there is no son to say "kadish" for a father, can a close grandson say it instead?

In the Gemara is written that consoling the mourner is forbidden with a few bare exceptions. The reason is that on Shabbat a person should not upset himself. Visiting the cemetery is obviously depressing for a variety of reasons. A grandson may say Kaddish for his grandfather even though his father is still alive, as long as his parents agree. Rabbi Ro'i Margalit