The last seven years of his correspondence make for distressing reading, as he hatches impractical plans for his financial salvation, and what promising opportunities arise are spoiled.
I would like to ask you two things regarding the bold passage of the above sentence. As I understand the sentence I would replace "what" with "how". The correspondence shows how promising opportunities spoiled. The usage of "what" does not make much sense to me. The second question concerns the verb "arise". Why is it in the form of the bare infinitive and not of the participle – "arisen"?