I do feel as if you're overstating the case somewhat. There is the lengthy, and generally offensive, monologue by the Dowager Duchess in chapter 3 - but this (to me) seemed an obvious case of showing the character as prejudiced rather than reflecting the opinion of the author or Wimsey. In another place I remember Wimsey was outlining the logic of the villain rather than offering it as his own.
I'm not sure that tar-baby was used as a pejorative term when this book was written, so I'm thinking the quote you offer was reference to the Uncle Remus story of the
Tar-Baby - the tar-baby says nothing no matter what Br'r Rabbit does.
If you read some of the links from earlier in his thread you will see there is reason to believe Sayer was not antisemitic herself. And note that antisemitism is part of the villain's motive, which seems to argue against this being how Sayer felt.
I believe that a lot of the over-the-top references to Jews in this book were there because this was her first novel and she was trying too hard. The dowager had to rattle on in an (unsuccessful) attempt to distract the reader from the fact that the solution to the story is being given away. Some other parts were to point the reader to circumcision without saying the word, and as hints as to why the murder took place. And some of the rest is, I suspect, a reflection of the language of the times; comments made because a Jewish character was part of the story, very little different to the casual sexism, racism and classism that is to be expected when reading older works (and that still hasn't been eliminated in modern works).
None of this necessarily makes it fun to read now, but I think it's a mistake to read the comments in this book as a reflection of Sayer's opinion. As I said in an earlier post, she also had a doctor confess to murder and made professional detectives appear ignorant next to an amateur, yet I don't imagine she was doing this as a result of her personal beliefs.