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"And the gates of Hell shall not prevail against you"
--Jesus to Peter

(The quote above is from a posting that's no longer viewable, apparently having been later deleted by a CathInfo  moderator, so I'm unable to provide a valid link.)

That is not an accurate quote.  The Douay-Rheims (and every other Bible I know of) says "shall not prevail against it", meaning the Church herself, not St. Peter himself in this case.

Ah, yes: Here we go again in CathInfo, arguing the wording of the Bible without any reference to the original Latin!  Actually, what the Vulgate says in Latin is the equivalent of English "her":

[18] Et ego dico tibi, quia tu es Petrus, et super hanc petram aedificabo Ecclesiam meam, et portae inferi non praevalebunt adversus eam. [*]

"Ecclesi·am, -ae" is plainly feminine, classically meaning an "assembly of the people", borrowed as a straightforward transliteration from the comparable Greek (where the word also seems to be feminine).  Thus the unemphatic demonstrative feminine pronoun e·a, -ius (e·am being its accusative-case) above, meaning "her".

The Rheims translators made an understandable concession to English and its natural gender, in which a "church" is a "thing", so that readers will indeed see the neuter pronoun "it":

[18] And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it. [*]

That aside, it seems to me that there's no compelling reason why the Rheims translators couldn't have used the feminine pronoun in English: Weren't the Catholic faithful of the time accustomed to the phrase "our Holy Mother the Church"?

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Note [*]:
<http://www.drbo.org/cgi-bin/d?b=drl&bk=[Matthew]&ch=16&l=18-#x>.

At the end of the day a non catholic can not be head of the Catholic Church , ...

But that's just one position, the Bellarmine position (unless you're Salza or Siscoe).  Other theories hold that a non-Catholic (aka heretic) must be removed from office by the Church, and the sedeprivationist thesis that he formally ceases to be pope but then must be removed materially.  This is an oversimplification.