If you don't want an hermeneutical analysis or similar interpretation of the text, then better get rid of it; and stick to the one sentence you are asking about. And/or come up with a parallel one. I do both here.
They stumble because they disobey the word, as they were destined to do.
As they were destined to do is an adverbial clause (see Grammar Monster and Thought.com)
As a writer you often have to explain to your reader where things happened, why things happened, when they happened, or how they happened. As you have seen, to answer such questions requires the use of adverbs, whether you use an adverb, an adverbial phrase, or an adverbial clause. (See Appositive Phrases, and Adverbial and Adjectival Phrases and Clauses, italics mine.)
Regarding the structure of your sentence, the adverbial clause can apply either to the closest preceding dependent clause, disobey the word or to to the complete independent clause preceding the adverbial. Thus, in the second option, they were destined to stumble because they disobey the word.
Compare the more pedestrian:
They fell off their bike because they got a flat tire, as I predicted they would (do).
As I predicted they would (do) is an adverbial clause.
I predicted either that they would
get a flat tire, or they would fall off their bike because they get a flat tire.
So, a grammatical analysis does not provide a conclusive reading of the sentence.