Firstly, why did the people not put this question to the mother of Imam Mahdi (as) so that her name would be clear for all?
It is a universal custom that whenever a person is introduced then he is introduced through the name of his father. The mother's name is rarely, if at all, asked. The reason for this is that the family continues through the name of the father and not the mother.
The family tree of the Holy Prophet (saw) goes back to Hazrat Adam (as), but how much do we know about his respected mother or grandmother. Information about them is extremely rare. So does this put a question mark on the prophethood of the Holy Prophet (saw)?
A custom not limited only to olden days, even today, we find families who are not forthcoming with the names of the women of their houses to the extent that even on the wedding card, they do not write the name of the bride and in its place, introduce the bride through her father.
There are some people who have passed before us who do not mention the full name of their daughters even while writing letters to them. For example, if one's daughter's name was Sakina, then one would write as Salaams to "S".
There are sufficient narrations from the Holy Prophet (saw) and the respected Imams (as) which prove that a child was to be born in the house of Imam Hasan Askari (as) - a child who would fill this earth with justice and equity, just as it would be rife with injustice and inequality. The government was equally aware of these narrations and hence the soldiers who guarded the house of Imam Hasan Askari (as) were given standing instructions to enter the house of the respected Imam (as) without permission if need be, and if a child was found with any woman of the house, the child was to be beheaded immediately.
Hence, it is possible that the Imam may have employed the services of many slave girls to protect the identity of the true mother of Imam Mahdi (as) in the event of the sudden and unlawful entry of government soldiers and the names which are being taken may be of any of these slave girls present in the house of Imam Hasan Askari (as) at the time of the birth of Imame' Mahdi (as)
We witness that one person can have more than one name. In fact some are given different names right from the beginning. Some names are historical, some introductory, some are those used to call a person with love and some are to describe a person's characteristics. This custom is not limited to any era, race or tribe. Then why is the robe of suspicion thrown only on the holy personality of the mother of Imam Mahdi (as), Janabe Narjis on account of the multiple names narrated in traditions?
This characteristic is found even in Allah, the Prophet, and Imams and even in Bab and Bahaullah - they have more than one name by which they are called and referred to. These names could be on account of their titles or their qualities and even their traits. Traditions narrate that Janabe' Zahra (sa) used to call Ameerul Mo'meneen Ali (as) by various titles as Abu Torab, Abul Hasan etc. Allah (swt) has also referred to his most superior creation, the Holy Prophet (saw) by numerous titles in the Quran. We find that Allah has referred to the Holy Prophet as Yaseen at some place and as Taha in another. Allah has not restricted Himself to call the Holy Prophet (saw) by his name, Mohammed Ibne Abdullah.
It is recommended that one should change the names of the slaves after purchasing them as per the laws of the Shiah Faith (Refer to the books of laws under the chapter of business). It is not necessary to completely wipes out one's past. Hence it is possible that a slave or slave girl could have more than one name. Janabe' Narjis apparently was a slave girl so it should not come as a surprise that she could have more than one name. Names like Sousan, Rehana, Seqal and Narjis were names common amongst slave girls.
As for the objection that why the mother of Imam Mahdi (as) was herself not asked this question, the answer is as follows. When Imam Hasan Askari (as) offered that Janabe' Narjis (as) could go back to her home, she implored and begged him that he (as) pray that she leave this world before him. (Behar, volume 3) Since the respected mother of Imam (as) expired at such a young age, it is unlikely that any narrator would have had an opportunity to ask her this question.
In a narration from Behar volume 13 page 4, Janabe' Narjis, while enumerating her past to Basheer Ibne Sulayman, outlines her genealogy as follows - "I am ….Then when I left the palaces of Rome in an ordinary dress and was arrested along with the Shaykh, I gave my name as Narjis since it was a common name amongst the slaves." The narration ends with the words of Imam Ali Naqi (as), "For surely she (Narjis) is the wife of Abi Mohammed (Imam Hasan Askari (as)) and the mother of the Qaem (as)"
Another point to note is that when Imam Hasan Askari (as) did not reveal the birth of his son Imam Mahdi (as), except to a chosen few, how is it possible that he would reveal the name of mother the respected Imam?
The Holy Prophet (saw) himself came in a dream to her and she accepted Islam in his presence. He (saw) also recited her nikah (marriage formula) with Imam Hasan Askari (as) who referred to her by her true name. Hence in the market place, she kept on providing different names with the intention that she would go only with the person who would take her true name.
Lastly we must note that not knowing the name of the mother of a person does not cause disrepute. If this were not true, then the greatness of many prophets (as) would come under question as we do not know the names of their mothers.