Who was in the wrong here?

This. Choosing the MT before could be defended because we'll it's likely the original Hebrew text.
With the DSS it has been proved the MT is a fake, it has some literary value in those few parts which it has in common with the LXX, because some expressions sound better in Hebrew but other than that there's nothing good in it.
Every Bible should use the LXX as the main structure of every Bible now that has been proved that the MT is Jewish bullshit. And people should have seen it coming. The redaction of that text was completed way after Jesus by the same dudes that were btfo into oblivion by Him.

Yep that's true. One might think at first if they werent afraid of messing with the scriptures and facing God's wrath, but wait they killed God Himself. Everything else is a kids play.

Catholics were even tolerant to them in the middle ages because we thought they were only those poor souls who rejected Christ and lived by the OT law.
We couldn't be more wrong. We weren't aware of the changes that had been happening to Judaism after the 1st century AD.
It was a converted Jew who casually mentioned the Talmud to the astonishment of all his monastic friends.
That same dude told that to the pope and he was surprised no one had ever heard of the Talmud. The winnie the poohers kept they're disgusting book hidden from the world for 600 years.
Then in a massive shit storm in France they admitted to the King that the Talmud could be defended but they couldn't and say it wasn't our Jesus and Mary lol.
More some fake gospels baspheming Christ were pread during the Middle Ages by the Jews themselves not even to mention the times they stole the Holy Eucharisty from churches to perform their satanic rituals.
Like Saint Augustine said: "they are the image of Judas, who sells the Lord for silver".
And the church fathers didn't even know half of the story.

how come jews claim only the torah (and not the entire tanakh) was translated into greek before Christ (so that septuagint was a greek trans. of the torah) if there are multiple quotes from other books in the NT that agree better with the LXX (right?)?

jews like to say only 5% of the DSS texts found were in greek, but it doesn't mean anything. The point is: does the LXX agree with the hebraic versions of the DSS over the MT?

Encyclopedia Britannica has this:

the fact psalm 151 is in the DSS and LXX and not in the MT might be enough