Double Negative with Parallelism

Question

I came across this long sentence in a novel, and would like to analyze it grammatically:
"I spent [...] time racking my brains for gems of Philosophy [....] , but I have come up with nothing that you could not (and probably already) have found in Plato, Socrates and the pages of Punch Magazine"

I know what the speaker is trying to say.
Now, the whole sentence above (given as a reference for folks who would rightly want the whole context) is not necessary for my question here, so I left out some words and highlighted a sub-sentence, for simplicity. The relevant part is:
"I have come up with nothing that you could not (and probably already) have found in Plato"

With the double negative, what is the parallelism structure ?
I think there is something wrong here with the tenses because, it requires "could not have found" and "would not already have found". The original sentence does not have "would", and the parallelism seems improper.

Answer 1

The sentence is flawed, but it's a matter of "faulty parallelism", not the use of could, which is perfectly OK.

I have come up with nothing that you could not have found in Plato ...

This says that it would have been possible for you to find the things he came with by looking in Plato. That's not quite the same thing as

I have come up with nothing that you would not have found in Plato ...

which says that you would certainly have found the things he came up with.

What is wrong here is that double use of have:

      (could not        )
 you -(and              )- have found in Plato ...
      (probably already ) 

It is permissible to employ conjoined constituents on the left with a common consituent on the right

      (might  )
 you -(and    )- have found in Plato ...
      (should ) 

In some circumstances it is even permissible (albeit deplorable) to use this structure across constituent boundaries, as here, where the conjoint elements participate in two constituents, both a verb and preposition phrase:

      (point to  )
 you -(and       )- this passage in Plato ...
      (depend on ) 

But in this sentence the shared have does not play the same role with respect to both of the conjoint left elements. In you could have found it is the infinitive complement of could, but in probably have found it is the finite second-person singular auxiliary. This jumps out if we recast the sentence in third person:

      (could not        )
 he  -(and              )- *have found in Plato ...
      (probably already ) 

There the have must be has to agree with the second conjoint element.

Answer 2

Forgetting about "(and probably have)" for now:

There are things that you could have found in Plato (ie the works of Plato), if you'd looked (which most people have not, hence the hypothetical nature of the statement).

There are things that the author has come up with.

When you subtract the first set of things from the second set of things, you are left with nothing.

Adding "(and probably have)" makes it less hypothetical since the author is saying that we probably have all looked through the works of Plato.