When the ex post facto clause is breached and the error is obviously plain, does this Constitutionally forbidden error require a reversal of a criminal conviction?

When a greater court (FC, COA) does not vacate a criminal conviction, where a plain error substantially and obviously prejudiced a defendant and violated that defendant's Constitutional rights? Would such be a grave miscarriage of justice, thereby calling into question the integrity of that court?

Does the fatal variance, based on the improprieties of language in the Count 2 charge, using "AND" in comparison to the jury instruction, thereby deprive the petitioner of his Constitutional rights granted by the 6th amendment "to be informed of the nature and cause of the accusations? And does this fatal variance also violate the 14th amendment of due process of law depriving petitioner of these vital Constitutional rights?

Does the fatal variance of the improprieties of language used by the petitioner's indictment using Count 2 charges any dict forme using "OR" CAUSE "AND" in comparison to the verdict, thereby denying petitioner choice and thereby igniting a unanimous verdicts?

Would it be appropriate to allow the testimony of a government expert witness, that has a preconceived sign of the cause of death where the precise sequence of the substances present in the deceased have been redacted and/or the risk is incomplete and inadequate, be used to sustain a conviction? And if not would such testimony be deemed worthless?

When the as facto elauee is nic hed ae Ee eit 2 '5 obviously plain, does this error, reguire a reversal of Seog! criminal Conv aha