Must the Kallah inform her Chasan that she is a Beula-not a virgin?
[It is forbidden for a Kallah who is not a Besula to withhold this information from her Chasan and have him obligate himself to a Kesuba of a Besula.] If this occurred, the [entire] Kesuba is invalid. Accordingly, a new Kesuba of 100 Zuz must be written as soon as the Chasan is informed. [Nonetheless, the Kiddushin remains valid even in such a case.]
Must the Misader Kedushin be told that she is a Beula if he is not the person who is writing the Kesuba?
There is no need to inform the Misader Kedushin that the Kallah is a Beuala if he is a different person than the one being Misader the Kesuba.
At what stage must a Kallah inform her Chasan that she is not a Besula?
This information is not to be divulged until the Chasan has shown deep interest in closing the Shidduch and getting married. Some Poskim rule that it is even forbidden to divulge this information on the outset, prior to the first meeting, or prior to a serious development of the Shidduch.
 See Maharsham 7:152; Eiyn Yitzchak E.H. 1:67; Igros Moshe O.C. 4:118; Kovetz Teshuvos of Rav Elyashiv 1:159; Even Yisrael 9:133
 Implication of Michaber ibid; Eiyn Yitzchak ibid 15; Igros Moshe ibid; Kovetz Teshuvos of Rav Elyashiv 1:159; Even Yisrael 9:133
The reason: As the Kesuba is invalid, and it is forbidden to live with a person without a Kesuba. [Eiyn Yitzchak ibid; Rav Elyashiv ibid; See however Even Yisrael 9:133] Likewise, not telling the Chasan is considered Gneivas Daas. [Rav Elyashiv ibid; Even Yisrael ibid]
Other opinions: Some Poskim give room for the Chasan to not be informed that his Kallah is not a Besula. [See Maharsham 7:152 who gave such a Heter in a time of need, under specific circumstances and conditions; See Even Yisrael 9:133 that it is not considered to be living without a Kesuba, as the Chasan is unaware of its invalidation and will be deterred to easily divorce her]
 This includes both the Ikkur and Tosefes Kesuba. [Darkei Moshe 67; Beis Shmuel 67:4; See Chelkas Mechokeik 67:5 who questions why the Tosefes Kesuba is also invalid]
 Michaber E.H. 67:5; Kesubos 11b
The reason: As this is a classic case of Mekach Taus [a transaction performed under false pretenses], in which case we invalidate the obligation. [Beis Shmuel ibid]
 Rama 68:9; Tur 68; Rambam
 Implication of Rama ibid, Rambam and Tur; Michaber E.H. 39:5 that if he accepted the Mum after discovery, the Kiddushin is valid; Haflah in Kuntrus Achron; Eiyn Yitzchak ibid 2-15 [See there for a thorough analysis on this subject, and his conclusion that according to all the Kiddushin is valid]; Rav Elyashiv ibid; See Even Yisrael ibid
Other opinions: Some rule that the Kiddushin is invalid and needs to be redone. [Tosafus Kesubos 11, brought in Beis Shmuel 68:24; See Eiyn Yitzchak ibid]
 Igros Moshe 118
 Igros Moshe ibid