No, that's all very interesting. I knew my arguments weren't seaworthy, I just wanted to provoke a linguist to further exposition. And it worked!
The only bone I'll pick:
"Ich werde den Apfel essen."
We see that "den Apfel" does in fact take the accusative, which means that "essen" is not acting as the direct object of the sentence.
Yes, but I was viewing the whole phrase "den Apfel essen" as the DO. So "den Apfel" taking the accusative was really my point. I guess I was viewing the complement structure as one cohesive phrase, as you pointed out.
And I may have used the term 'predicate' wrong. I was under the impression that it's just the verb attached to the subject, so thanks for clearing that up.
But my point stands that the modal "werden" gets conjugated as the main verb (the one attached to the subject, whatever the correct word for that is), while "essen" gets changed back to the infinitive and moves to the end of the sentence to get attached to the DO.
That's where I was coming at it from, but I apparently didn't use the correct terminology. So that's my bad.
Also, sorry for the delayed response. It took a while for my brain to process it and decide what to say. Thanks for the intellectual stimulation!