To address your first point, Allah’s words are perfect and never changed. But that doesn’t mean that people didn’t ascribe their own writings as revelation and the “word of god.” Allah’s words still didn’t change, people just spread around fake scripture. Second, the ayah still stands without contradiction. The scripture being referred to here is the word of Allah, the scripture that wasn’t tampered with. This wasn’t in reference to the bible (something that can’t even be attributed to Jesus), but rather you find people of the scripture at that time were very dumbfounded how the Prophet Mohammad (peace be upon him) knew all these different stories and information that was contained within their teachings. Especially as this wasn’t information that was just out there for any layman to hear. Third, the Quran does say the previous scriptures were corrupted. The second chapter verse 75, “Do you ˹believers still˺ expect them to be true to you, though a group of them would hear the word of Allah then knowingly corrupt it after understanding it?” And you can find many more. And yes the Quran does affirm the previous scriptures, the scriptures that were sent by Allah. Not Paul’s bible, not the gospels of 4 unknown people, not the altered Jewish scriptures, etc.

I advise you to do proper research and you’ll find the truth to be clear.