You make a very good point that the arguments between Catholics and non-Catholics on these threads WRT sola Scriptura really is, when you get down to it, a dichotomy between which is the authority for the Christian faith. It is either Scripture or the Church. As you put it, sola Scriptura versus sola ecclesia. If the magesterium is infallible in everything it declares to be de fide, then we don't need infallible Holy Scriptures. But if the Magesterium derives its authority from Scripture, as so many Roman Catholics here assert as well as their Catechism, then it must also be infallible to be counted on to give them this assured status. So, then, if Scripture is infallible - as it is alone divinely-inspired - then it is the true authority ABOVE the Church magesterium.
Your conclusion is correct.
Not to mention that Christianity itself was not founded upon the premise of an assuredly infallible perpetual magisterium instrumentally assured via formal decent (though SS affirms ordination), as that would have required all to have submitted to those who sat in the seat of Moses, (Mt. 23:2) and thus there would have been no church. As also argued here: http://www.freerepublic.com/focus/news/2834915/posts?page=131#131, http://www.freerepublic.com/focus/news/2834915/posts?page=157#157