Double relative clause or something different
I assume, throughout, the state of things which, where the labourers and capitalists are separate classes, prevails, with few exceptions, universally.
Are those true or false?
The state of things prevails universally. The labourers and capitalists are separate classes in this state of things.
So can I rewrite?
The state of things which prevails, with few exceptions, universally and where the labourers and capitalists are separate classes.
You can rewrite the sentence, but not in the way you did.
First strip out all of the nonrestrictive information:
✔ I assume the state of things which prevails universally.
That sentence is fine.
Although it looks a little odd, the state of things is a singular subject, so it takes a singular verb (prevails).
On the other hand, if you strip out the nonrestrictive information in the rewritten sentence, you are left with something ungrammatical:
✘ The state of things which prevails universally and where the labourers and capitalists are separate classes.
That is a sentence fragment. It's a noun phrase without a subject or a verb for the missing subject. It might be stylistically acceptable in some contexts, but not as a standalone sentence.
If you were to say that to somebody, they would likely reply, "Yes, and … ? What about that state of things?"
As for asking if those things are "true or false," that's not a question that grammar can address:
- The state of things prevails universally.
- The labourers and capitalists are seperate classes in this state of things.
Those are both grammatical statements. But if they are true or not is not something that can be answered by the English language itself.