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A plaintiff won a federal jury verdict in a products-liability case. Before trial, the district court denied the defendant's Rule 12(b)(6) motion on a purely legal preemption issue. At trial, the defendant objected to one expert's causation opinion as unreliable, but did not object when the court later admitted a cumulative internal email. The defendant also proposed a jury instruction on misuse, objected on the record when the court refused it, and then lost the jury verdict.
On appeal from the final judgment, the defendant challenges the preemption ruling, expert ruling, internal-email ruling, rejected jury instruction, and the jury's factual finding of causation. The plaintiff argues that all errors were either unpreserved or harmless.
How should the appellate court analyze the issues? Discuss preservation, standards of review, and harmless error.