That's weird, I don't know why the production amount would be off.
Corruption even for Barbarians can be reduced by buildings in their cities.
I hadn't thought about maintenance, that might be a problem. Presumably the barbarian player is being charged maintenance for all barbarian units on the map.
Yes, I think the Barbarian player pays for all Barbarian units on the map.
Unfortunately the multiplayer tool cannot detect a save file started with an exe boosted with the Flintlock Mod.
But surprisingly the multiplayer tool can detect a save file started with the Antal1987 exe attached to CCM 2.5. So each modder who has installed CCM 2.5 properly (as this mod also holds the multiplayer tool), can do the following (and of course other) tests without problems:
When loading a save file created by the R16a version of the Flintlock mod with the Antal1987 exe and allowing to play the Barbarians with the multiplayer tool, one can see what happened with the Barbarians in the last turn when being played with the R16a version of the Flintlock mod (let´s hope there are no distortions caused by this method).
A view into the domestic advisor shows, that the Barbarians pay upkeep costs for 43 units, but a view into the Barbarian cities shows, that there are only 12 Barbarian units in those cities. So it seems, the Barbarian player is being charged for all Barbarian units on the map.
... and the Barbarian player is also charged for the maintenance costs of buildings in the Barbarian cities.
Another view shows what tech research the Barbarians had started under R16a:
They started to learn the Alphabet in that scenario.
Another view in Debug Mode at the end of era 4 of this scenario (The Far distant Future) shows the era 4 city graphics of the American culture group, that I have added to the Conquests folder, so for the Barbarian capital Neapolis only the size 2 graphics.
When continuing the game with the Antal1987 exe and the multiplayer tool, the Barbarians were even able to negotiate a peace treaty with Caesar !
All in all, at present the maintenance costs for Barbarian units and buildings shouldn´t be a big problem at least for CCM, as a chain of Barbarian units without unit support costs can be easily created and CCM buildings have no maintenance costs. More difficult could be to assign a special starting tech only for the Barbarians (but I haven´t tried this yet).