The Kollel wrote-
>>(RABBI ARI MARYLES (Chicago) points out in his notes to SEFER TORAS SHIMON
by the Yoruslaver Rebbe, Rebbi Shimon Maryles m'Yoruslav, that "Zachur la'Tov"
is not the highest form of praise, for the true Tzadikim have a much higher
form of praise, as we say in the "Shoshanas Yakov" prayer after the Megilah
reading, "Beruchim Kol ha'Tzadikim" -- a Tzadik is "Baruch," while
Charvonah, who did not reach that status, was only "remembered for the good.")<<
However, when we want to 'speak well' of Tzaddikim who are in Olam Ha'emes we say Zecher Tzaddik Livrocha. We speak in general of Razal, Chazal etc. with their remembrance as a brocha. I have never understood this concept - why should the memory of someone be for a brocha, rather than their teachings for example.
What we say when we refer to Tzadikim who have passed away accurately reflects what we say in the prayer of "Shoshanas Yakov" regarding Tzadikim -- that they are "blessed," as opposed to just "remembered for the good." Zt'l and Z'l mean, as you wrote, "Zecher Tzadik l'Vrachah ," and "Zichrono l'Vrachah ," and thus we mentioned that they are blessed, which is a higher level than just saying may they be remembered "la'Tov -- for good."
Regarding your question about why we say "Zt'l" altogether, this is based on the Gemara in Yoma (38b) which discusses the phrase, "Zecher Tzadik l'Vrachah" as it appears in Mishlei (10:7), and its application to Tzadikim after they have passed away. For details about when to use such a phrase, see TA'AMEI HA'MINHAGIM, page 471-473 (footnotes). For the meaning of the phrase, see the commentators (Rashi, Ibn Ezra, and others) to the verse in Mishlei. (The Ta'amei ha'Minhagim cites the DIVREI EMES (the Chozeh of Lublin) who says that when a person says "Zecher Tzadik l'Vrachah," it means that by remembering the Tzadik (which is an honor to him), Hash-m will bless the person who remembered the Tzadik.
Y. Shaw