a. Is the special supplication to be recited after drinking Zamzam water or before it? b. When glorifying God in the morning remembrances or after prayer, should the remembrance be repeated or continued if the speech is interrupted? c. Which is better: to pray Witr in the Haram after Isha or to delay it until returning home?

Supplicating (Du'a) when drinking Zamzam water is permissible either before or after drinking it.

As for interrupting the tasbih (glorification of Allah) to respond to someone who speaks to you or to guide a questioner, there is no harm in that. The one remembering Allah then returns to the tasbih and completes their count.

As for the Witr prayer, it is preferable to perform it at home, based on the Prophet's (peace be upon him) saying: "So pray, O people, in your homes, for the best of prayers is a person's prayer in their home, except for the obligatory prayers."