ubi: I am not sure why you think Tisha B’av is an incorrect date to commemorate the shoah. After all, the Bobover Rebbe TZATZAL and Reb Schwab TZATZAL each wrote a kinah for it.
About 35 years ago I attended a Yom Hashoah talk given by Rebbetzin Esther Jungreis A”H and YBLCH her brother Rabbi Jungreis. I don’t know if Chaf Zayin Nissan is the appropriate. All I know is that it was an hour that I have never forgotten.