Yet, the DSS are closer to the Septuagint than to the Masoretic text - and the differences are more than just spelling errors.
No, that is not correct. Please cite your source. Have you even read the Isaiah scroll? It is not more in line with the LXX than the Mesorah.
The Masoretes were more meticulous with the text that the scriptoriums that worked on the Greek Scriptures. The thousands of variants (some quite divergent) are proof enough. There is only one reason to mess with the Mesorah - people who don't like what it says.