Mormonism certainly holds to absurd teachings, from her equating other works to Scripture to, making her ministers priests, to her doctrine of eternal progression, and even a heavenly mother.
Yet she operates out of the same sola ecclesia model as Rome, with the "church" effectively being the supreme infallible authority on what Truth is and means, and with a "living prophet" rather than a pope.
Certainly you can make a case for what Scripture, history and tradition teaches as supporting Rome, versus how Mormonism interprets them, but if objective examination of these evidences is your basis for assurance of the veracity of RC claims and teaching, than this is the same basis as evangelical type Christians have historically held to, and thus contend against both Rome and the LDS.
In doing so we find that both resort to the argument that their church is uniquely qualified to define what the evidences teach, and RCs esp. dismiss conclusions contrary to theirs as being invalid due to being the result of reliance upon fallible human reasoning, and being fallible, thus one needs an infallible magisterium.
Therefore, while you hold that the testimony of the Catholic Church overcomes Mormonism, it seems that the argument is that the testimony of the Catholic Church as being the historical instrument and steward of Scripture renders her to be the infallible authoritative authority on what Scripture and other evidences mean.
That would be because Christ told us so: in Matthew 16:18...and in John 14,
But such texts are interpretive, and (among other teachings) did not even enjoy the "unanimous consent of the fathers," and Scripture and even Catholic scholarship provide evidences against the early church corporately looking to Peter as its exalted supreme infallible head in Rome.
Thus again, unless your assurance rests upon Scriptural substantiation as establishing Rome as the OTC, it would seem your argument is that an infallible interpreter is necessary for assurance of Truth, and that Rome is that interpreter, or declarer, in the light of her historical instrumentality and stewardship of Divine revelation.
It is not complicated: I believe the testimony of the Church because I find it credible. I do not find the claims and theological pretenses of Mormonism or Protestantism credible.
And i do not see the NT as testifying to Peter being the rock, versus the alternative understanding, that "this rock" refers to the rock of this faith confessed by Peter that Christ build his Church, and thus upon Christ Himself, which your catechism allows for, which understanding some of the ancients concurred with.
And that the promise to teach all (Jn. 14:26) and to lead into all Truth is realized thru the Scriptures as being supreme and what is based and established upon it, and culminating in glory. (1Jn. 3:2)
But insofar as interpretations differs with Rome then RCs often assert that "the (Roman) Catholic church gave you the Bible, she knows what it means," and thus my questions as to the real basis for your assurance of Truth, and the reasoning behind it.
Indeed, but Protestantism is only marginally better. For example, instead of seeing in the verses I pointed to the inescapable meaning of continuing inspiration of the Church which "abides for ever" and the promise of victory of the Church over Hell, you sidetrack to the issue of "rock", because that is where you have a template for arguing. Yet on the papacy, too, the scripture is with us: clearly by rejoicing over the confession of Peter, renaming him after Christ's own attribute, and promising the keys to heaven Christ meant to elevate Peter as a person, not some abstract faith. Luke 22:32 clearly gives Peter a role of safeguarding the faith of the other apostles. With all this you can argue around the edges but you cannot un-write the Holy Scripture. It simply doesn't teach what you imagine the teaching of Christ to be. Similarly your interpretation of John 14 verses does not match even formally what the scripture says, because in the scripture Christ promises the leadership of the Holy Spirit in the utterances of the Church "for ever", and not till such time as the New Testament is written and canonized.