[Grammar] Is 'which she knew was' Grammatically Correct?

[Ulysses]

In my sentence, I use the phrase 'which he believed was' in the following sentence below and I'm not sure if I'm using it correctly since it does not follow a subject verb structure.

He restricted his ideas for the play to those he thought were politically correct which he believed was the best thing to do.

 

[mmasny]

In my sentence, I use the phrase 'which I knew was' in the following sentence below and I'm not sure if I'm using it correctly since it does not follow a subject verb structure.

He restricted his ideas for the play to those he thought were politically correct which he believed was the best thing to do.

I'm not sure, but I think there should be commas in this sentence to make it more comprehensible.
He restricted his ideas for the play to those he thought were politically correct which, he believed, was the best thing to do.

 

[Ulysses]

I'm not sure, but I think there should be commas in this sentence to make it more comprehensible.
He restricted his ideas for the play to those he thought were politically correct which, he believed, was the best thing to do.

Wouldn't that make it a comma splice when you separate verbs?

And I have something else, how would it work if believed was changed to thought as in:

He restricted his ideas for the play to those he thought were politically correct which, he thought, was the best thing to do. <- Doesn't sound right?

 

[Raymott]

In my sentence, I use the phrase 'which he believed was' in the following sentence below and I'm not sure if I'm using it correctly since it does not follow a subject verb structure.

He restricted his ideas for the play to those he thought were politically correct which he believed was the best thing to do.

I would write it like this:
He restricted his ideas for the play to those he thought were politically correct, which he believed was the best thing to do.

This is a non-restrictive 'which' clause which relates to the whole preceding part of the sentence, and so needs a comma.
On the other hand, mmasny's version is not wrong; it parenthesises 'he believed'.
If you read the other current 'Comma Use' thread, you will a suggestion that both should be used, ie.
politically correct, which, he believed, was the best thing to do.

Personally, I don't think 'he believed' needs to be parenthesised.