Only as you “called” me … I will give a quick reply without reading the full “story so far” … you have a backup so Upgrade with Format data seems the clean way to go.
One might read the Updater script with the ROM in /meta-info or such to check my memory of there being some special requirements ??
Perhaps we don’t take this entirely literally
… but the current recovery may not fit with the current sideload and updater script.
Are we jumping a13 → a15 at that time?