But Spector DID diversify his sound - the Beatles and solo albums, the Dion album, Ramones, quite different from the wall of sound he pioneered. And he did cut a few great albums, too, it wasn't all about singles, try "Love is all we have to Give", by Sonny Charles & the Checkmates from '69, an overlooked masterpiece.
I don't believe Spector was in the studio during the recording of "Let It Be" but was handed the unmixed tapes the band gave him.
You're right - he was only called in after the sessions to see if he could inject some energy & enthusiasm into the songs. By the way, in the famous NME interview that I mentioned in my first post in this thread he claimed in a not too subtle way that he had produced the fab four all along during the 60s.
Oh, the megalomaniac!
He
was actually on the plane when the Beatles came to the US. You can see him just behind the guys when they leave the plane on some of the footage from back then.