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

QUESTION(S) PRESENTED 1. 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? ; 2. Did the government agent's testimony severely prejudice the defendant's substantial rights and caused reversible error? ; 3. Did the District Court err in allowing untendered expert testimony that was used to establish guilt? Il.