"We know that all the power of the mother of God is derived from the merits of her Son." -- 
http://haydock1859.tripod.com/id329.html
		 
Interesting you found the Roman Catholic apology / explanation 
for the mistranslation so quickly.
By this I presume you are one of the many Roman Catholics that 
stalk this site, only under yet another sock-puppet.
So noted.
Since you link it, here is the full quote on this problem:
| [FONT=Times New Roman,Times,serif]                            Ver. 15. She shall crush. Ipsa, the woman: so divers of the fathers read this place, conformably                            to the Latin: others read it ipsum, viz. the seed. The sense is the same: for it is by her seed, Jesus Christ,                            that the woman crushes the serpent's head.  (Challoner) --- The Hebrew text, as Bellarmine observes, is ambiguous:  He mentions                            one copy which had ipsa instead of ipsum; [in the LATIN]  and so it is even printed in the Hebrew interlineary edition, 1572,                            by Plantin, under the inspection of  Boderianus.
 
 Whether the Jewish editions ought to have more weight with  Christians, or                            whether all the other manuscripts conspire  against this reading, let others inquire. The fathers who have cited the  old Italic                            version, taken from the Septuagint agree with  the Vulgate, which is followed by almost all the Latins; and hence we  may argue                            with probability, that the Septuagint and the  Hebrew formerly acknowledged ipsa, which now moves the  indignation of                            Protestants so much, as if we intended by it  to give any divine honour to the blessed Virgin Mary.
 We believe,  however, with                            St. Epiphanius, that "it is no less criminal  to vilify the holy Virgin, than to glorify her above measure." We know  that all                            the power of the mother of God is derived  from the merits of her Son. We are no otherwise concerned about the  retaining of                            ipsa, she, in this place, than in as  much as we have yet no certain reason to suspect its being genuine. As  some words                            have been corrected in the Vulgate since the  Council of Trent by Pope Sixtus V. and others, by Pope Clement VIII. so,  if,                            upon stricter search, it be found that it, and not she,  is the true reading, we shall not hesitate to admit                            the correction: but we must wait in the mean  time respectfully, till our superiors determine. (Haydock)
 Kemnitzius  certainly                            advanced a step too far, when he said that  all the ancient fathers read ipsum. Victor, Avitus, St.  Augustine, St. Gregory,                            &c. mentioned in the Douay Bible, will  convict him of falsehood. Christ crushed the serpent's head by his  death, suffering                            himself to be wounded in the heel. His  blessed mother crushed him likewise, by her co-operation in the mystery  of the Incarnation;                            and by rejecting, with horror, the very first  suggestions of the enemy, to commit even the smallest sin. (St.  Bernard, ser.                            2, on Missus est.) "We crush," says  St. Gregory, Mor. 1. 38, "the serpent's head, when we extirpate from our  heart                            the beginnings of temptation, and then he  lays snares for our heel, because he opposes the end of a good action  with greater                            craft and power." The serpent may hiss and  threaten; he cannot hurt, if we resist him. (Haydock)
 
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We don't seriously doubt that the error, and the dogma which follows it,
was originally an innocent one, one of thousands in the early Latin 
translations (freely, locally and individually done by Latin Christians 
throughout the Empire). 
We don't challenge that some early fathers (writing in Latin) 
tended to read 'she' instead of 'it' or 'he', using the Old Latin copies (c. 200-300 A.D.)
uncritically.
There can be only one true reading, and since the Hebrew is hardly 
as ambiguous as the Latins claim, there is little doubt what it is.
There are certainly some textual difficulties in Hebrew as well as the Latin, 
and Greek (for the NT), but the Latin translations are certainly one further 
step removed from the original as found in the majority of manuscripts, 
both Greek and Hebrew.
This really isn't one of those textual critical cases, but rather just 
some historical sloppiness in translation from Hebrew to Latin.
The Mariolatry that follows Latin translations is more sinister.