At this point I have tried all I could but do not know what else to try and hoping I can find a solution to these challenges.
Yeah, all points strongly suggest a corruption in the wallet's derivation of the latest private keys and address (
after the 5-year-old transactions)
Your only chance of recovery is if it's somehow associated with the other backups that you might have used in Mycelium.
If it's caused by random flipped bits due to the app/unreliable storage or hardware, it's as good as lost.