In the first example, rules for reported speech allow for the quotation mark or exclamation point within the quote to stay. The Chicago Style Guide provides this example in 6:10: Other punctuation in relation to closing quotation marks:
“What’s the rush?” she wondered.
If the question mark or exclamation point is part of the quoted material, put it within the quotations. If the sentence continues from there, let it. The exception is a period, where a comma would be used instead. (Wikipedia affirms this with the Merriam-Webster Guide to Punctuation as a contributing source.)
Examples 2 and 3 are flawed. This stylistic rule involving reported speech does not persist in other contexts. Instead, commas should be used if you're distinguishing elements in a sentence; if a question or exclamation is necessary, appropriate structural changes should be made to distinguish one sentence from another. For example, here is one way to revise your sentences:
"Would I do that? Because I don't think I would." ("Because" begins a new sentence.)
In a more complex sentence, do exclamation and question marks replace
commas? This would mean they don't end the sentence. (Clarify "meaning" into subject and verb to begin the sentence.)