Did the testimony of a government agent, having not been tendered as an expert witness after interpreting and explaining events that he DID NOT observe, be potentially or likely to be seen by the jurors in a criminal trial as substantial to prove "beyond a reasonable doubt" a persons guilt because other direct evidence of involvement was unavailable to them and greatly influenced their decision?

Did the government agent's testimony severely prejudice the defendant's substantial rights and caused reversible error?

Did the District Court err in allowing untendered expert testimony that was used to establish guilt?

Did the testimony of a government agent, not tendered as an expert, influence the jury's decision on guilt beyond a reasonable doubt?