A very good question actually, but before i answer that it seems that the RC premise is that an infallible interpreter is necessary to authoritatively determine what Scripture all consists of and its meaning, and that Rome, being the steward of Divine revelation, and inheritor of promises of God's presence and preservation, and having historical descent, is therefore that infallible interpreter. Is this a correct argument?
Yes and no. God gave the Catholic Church the authority to determine the canon of scripture. As far as infallible interpreter the Catholic Church has only infallibly defined somewhere between 5-8 verses. Now that being said; anyone may have a different interpretation as long as it doesn't contradict the Church's teaching.
and that Rome, being the steward of Divine revelation, and inheritor of promises of God's presence and preservation, and having historical descent, is therefore that infallible interpreter. Is this a correct argument?
Also yes and no. See above. The Church is the divinely appointed steward (Excellent choice of words, seriously), but does not infallibly interpret ALL scripture.