You do seem to have forgotten, though, that even those who believe in predestination to election hold to the same gospel of salvation by grace through faith in Jesus Christ alone apart from works. The difference is in whether grace is "irresistible" or not. But this is hardly a proof in the "unworkability" of sola scriptura. Many theologians over the centuries have discussed the concept of election and predestination and many NEVER came to full and total agreement about it. I highly doubt even those who comprise the current Magesterium agree completely on the subject. Has the Roman Catholic Church even made a declarative statement WRT predestination and election?
It seems like you are insisting that if people don't agree on EVERYTHING then they can't be unified still on the major things. I know this is untrue. Many on these threads may disagree on this doctrine - and if we're honest - we should agree that Scripture is not precise about this with some Scripture seeming to agree with the idea while another disputes it. My take is that probably BOTH roads are true and we are unable in our finite human minds to get our "arms" around how it all works. But your point trying to prove that an "infallible" magesterium is necessary to know truth fails because, if that were true, we wouldn't need to have a Bible. If God really gifted a select group to have an "inside line" on all that is true about God then he didn't need to go to the trouble of putting together divinely-inspired Scripture. Infallible truth could have just been handed down from one infallible teacher to another over thousands of years and no one could have or ever needed to question it - EVER. But God didn't do it that way, did he?
Your lacking a petal or two on your TULIP. For some, both grace and faith are completely and entirely contingent and dependent upon being born elect. Salvation by election. Unlike your belief.
Very unlike your belief.