Will this mod replace the standard trains? What happens if you've placed one of the upgraded locos and later for some reason you have to uninstall the mod? Could there be provisions for them to be replaced by the standard locos/wagons or will they just get deleted with all the resources?

Also, is there a way to bring back the splitting on the fuel wagons?

Base locomotives, wagons and fuels have their stats adjusted.

When uninstalling the mod all additional locomotives and wagons simply vanish.
Migrating back to base entities is not possible without an additional uninstall mod containing a migration script.

No, there's no way to change splitting fuel wagons in a mod.