Selon la légende, des spectres hantent les salles. J'ignore ce qu'il y a de vrai là-dedans, mais...

In this sentence, there are two things I cannot understand:

1. Why the need to say "j'ignore ce que {I don’t know what}" instead of "j'ignore si {I don’t know if}"?

2. I was under the impression that the phrase "il y a du vrai là-dedans" is a fixed expression with the meaning of "there’s some truth in that". So why "de vrai" rather than "du vrai"?

Selon la légende, des spectres hantent les salles. J'ignore ce qu'il y a de vrai là-dedans, mais...

In this sentence, there are two things I cannot understand:

1. Why the need to say "j'ignore ce que {I don’t know what}" instead of "j'ignore si {I don’t know if}"?

2. I have read somewhere that in a question sentence or in a si {if/whether} clause, French does not make much distinction between "any {de}" and "some {du}". So why the need to say "de vrai" rather than "du vrai"? Is it equally correct to use "du vrai" in this instance?