Q. Can you say a shloshim hesped thirty days before Yom Tov for a community Rabbi?
Do you recite E-l Mole Rachamim?
If permitted, why is it different than others?

A. Shulchan Aruch (O.H.547: 3) rules that one should not eulogize the departed within thirty days of an incoming Yom Tov. The reason, explains the Talmud (Moed Kattan 8a) is not to raise feelings of sadness close to days of joy or to avoid spending (for the hesped) funds necessary for Yom Tov.
However, Horav Shlomo Miller’s Shlit’a opinion is that it is permitted to eulogize within or at the shloishim day after the demise, for someone who was a Talmid Chacham or recognized by his outstanding communal work and abnegation for the benefit of the rabim. Stressing, that the purpose of the hesped is not to awaken sadness and sorrow, rather to encourage others to follow and emulate the greatness of the niftar.
If the minhag of this congregation is to recite an hazkara at shloishim, it can also be said.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a