May an Avel go to Shul?
An Avel within Shiva may attend Shul on the night of Tisha B’av, and by day until the completion of the Kinos. [He is not to go to Shul for Mincha.] This applies even within the first three days of Aveilus. However some Poskim are stringent and rule that if the Avel is within the first three days of mourning, he is not to attend the Shul. Others say he is to only attend during the day and not at night.
Onen: An Onen does not go to Shul on Tisha B’av until after the burial. [However, Admur rules that a son which is an Onen on his father may go to Shul and say Kaddish for his father. Practically, so is the Chabad custom. The Rebbe said Kaddish while he was an Onen for his mother, Rebbetzin Chana, and his wife, Rebbetzin Chaya Mushka. Nevertheless, the Onen is to only participate in the Kaddish and is not to recite Kinos or any of the prayers.]
Does an Avel Daven for the Amud on Tisha B’av?
 559:6; Piskeiy Teshuvos 559:7
 Michaber ibid
 Ruach Chaim 559:2; Kaf Hachaim 559:56
 M”A 559:8; Elya Raba 559:18; Derech Hachaim 4; Kitzur SHU”A 124:4; M”B 559:24
The reason: Although, in general, an Avel may not leave his house within the first three days even to visit another Avel, nevertheless, in this case it is different, as on Tisha B’av everyone is an Avel. [M”A ibid; M”B ibid]
 Rabbeinu Yerucham brought in M”A ibid and M”B ibid
 Rav Shlomo Kluger, brought in M”B ibid
 M”B 559:24 in name of Elya Raba
 Admur 71:1 in parentheses; Taz 71:2; Misgeres Hashulchan 341; The following Poskim agree to this ruling if the Chevra Kadisha has taken over the proceedings of the body: Beir Heiytiv 71:4; P”M 71 M”Z 2; Chochmas Adam 153:1; M”B 71:7
 The reason: As this honors ones father. [ibid] This ruling of Admur is placed in parentheses possibly due to the other opinions mentioned above that argue. [Chikrei Halachos 5:50]
 Igros Kodesh 3:199; 4:107 [printed in Shulchan Menachem 5:294]; Custom of Rebbe brought in Nitei Gavriel 17:1 [p. 113] and Shulchan Menachem p. 265
 Shulchan Menachem ibid footnote 21
 Pashut; See Nitei Gavriel 55:19
 So is the worldly custom. [Piskeiy Teshuvos ibid]
 Siddur Yaavetz