1) This is indeed what Rav Shmuel bar Rav Yehudah meant when he said that any Arusah may not eat Terumah because of Simpon; he meant that it applies both to external blemishes and to internal ones. The same applies to a Shifchah since he may want to marry her off to his favorite Eved. A man is different, because internal belmishes are not crucial for him. The Gemara (end of 57b) is not referring only to a Shifchah; it is referring to any girl after Erusin.

2) I tried to say in the above reply that there is a difference between a male Eved and a Shifchah. However, I saw in Kidushin 10b-11a that the Sugya discusses a Shifchah eating Terumah and uses there (top 11a) the idea that there is no Simpon in Avadim because the master only needs the Eved for work, so we see that there is no difference between a male and female in this respect.

Instead, to answer this question, I think one must say that even though the Mishnah in Kesuvos 40a states that a Besulah Shifchah is worth more than a Be'ulah Shifchah, nevertheless it is not considered a blemish if she is a Be'ulah. It is an advantage for her to be a Besulah but it is not critical. So we do not have a Kol she'Ken anymore, because even if we say that Mumin d'Gavai are worse than being a Be'ulah, a Be'ulah is not actually a Mum.

3) We should look at the Mishnah in Avos 2:7: "Marbeh Shefachos Marbeh Zimah" -- the more Shefachos there are, the more immorality there is. A Shifchah is usually on a low level of morality. The Gemara in Gitin (beginning of 13a, just before the Mishnah) tells us that an Eved does not want to go free because as long as he is an Eved, a Shifchah Kena'anis is permitted to him. He likes this because she is willing to do with him whatever he wants and behaves in a Peritzus way with him.

4) I think this strengthens the second answer I gave above. One does not expect to find that a Shifchah Kena'anis is a Besulah; that means that it cannot be considered a Mum if she is a Be'ulah.

5) It seems to me that when the Mishnah in Kesuvos 40a states that one estimates the damage done to her according to the value of a Shifchah sold in the marketplace, and the Gemara there (40b) states that the master wants to marry her off to his favorite Eved, this is in a sense a sort of Mashal. What I mean is that it is a way of measuring damages. There is a rule, "Ein Damim l'Ben Chorin" -- a free man has no monetary value (see Magid Mishneh, Hilchos Nizkei Mamon 10:14). Therefore, we must measure damages according to the scale of an Eved, which is why the Shifchah who is a Besulah is mentioned, but that does not mean that the latter scenario is at all frequent.

(See Teshuvos Shevus Yakov 1:174 who writes that the negative things said by Chazal about Shefachos apply only to a Shifchah Kena'anis. For a Shifchah Ivris we say that every Jew has a Chazakah of being upright and "kosher.")

6) I found that this question is asked by a Sefer cited in Otzar ha'Chochmah. Sefer "Kach Hi Darkah Shel Torah" (Valdberg), vol. 1, page 42b, #30, on Kesuvos 57b, asks that the buyer of a Shifchah does care that she should not have an internal blemish because he wants to make a Koras Ru'ach for his servant!

He answers that there is a distinction betwen when a man gets married himself and when he buys a Shifchah to please his favorite Eved. When a man choses a wife for himself he is particular that she should have no internal blemishes even if he did not specify this condition before the marriage. When he buys a Shifchah it is only if he makes a specific condition that there should be no internal blemish that we say that he is Makpid on it even b'Di'eved.

7) There is another way of answering this question, according to Rashi to Kesuvos 40b, DH Koras. Rashi writes that the Nachas Ru'ach that the master wants to do for his favorite Eved is to give him the taste of a Besulah. So, according to this, the Mumin d'Gavai might not matter so much, because the most important thing is that she should be a Besulah for the first Bi'ah.

Kesivah v'Chasimah Tovah,

Dovid Bloom