"and what promising opportunities arise are spoiled"

Question

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"?

Answer 1

Compare OP's cited text with this one, which may be clearer...

There's not much food in the office fridge, and what there is belongs to the boss.

The implication of and what [food] there is [in the office fridge] is always that there's not very much, and that even that relatively small amount is worth less than one might have hoped.

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Thus in OP's context it's being asserted that very few "promising opportunities" actually arise, but even the ones that do aren't worth much because the correspondent always "spoils" them in some way (i.e. - he fails to take advantage of occasional chances to improve his financial circumstances).

Answer 2

and what promising opportunities arise are spoiled.

The reason arise is used instead of arisen is because the timeframe is anchored in the past and we are observing the situation as if we were there at that time

he hatches plans
opportunities are spoiled

to use how, the sentence would change to

how promising opportunities that arise are spoiled