The valid reason to deny a repair is in the purchase agreement you make which states that you are not to open the machine, only a qualified repair person can. You take full responsibility understanding that when buying the product. So Apple doesn't even need a reason, it's a pre-requirement for you buying the machine.
They may only void the warranty, not the right to a repair. A repair attempt of a machine can not and is not part of any implied contact when you purchase something.
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Well, what should I do? I do have real-world knowledge on the subject. Should I pretend I don't?
Don't say you aren't qualified and then explain why you are. It sounds like you are, so lead with that.