1): VH battle difficulty is not recommended. It warps things to the point that a unit of AI-controlled levy spearmen can beat your legions in one-on-one combat. Medium is the recommended difficulty, and does not detract from the enjoyment from playing a campaign at all.

3): The three nomadic factions in EB are as follows: Sauromatae (Sarmatians, replacing Scythia), Pahlava (Parthians) and Saka Rauka (New Indo-Scythian faction, they're the red faction in the utmost northeastern corner of the map). Horse archers are bread and butter for these factions.

The non-nomadic factions that start out with access to horse archers are as follows: Getai (Dacia), Hayasdan (Armenia), Arche Seleukeia (Seleucids) and Baktria. These factions rely primarily on infantry, with horse archers being used primarily as auxiliaries. And yes, the Getai replace the Dacians, not the Thracians.

Balance-wise, the Romans are good, but if you get careless you'll find your troops being massacred by your 'barbarian' enemies. The Lusotanna in Iberia are particularly deadly, since a good half of their roster have armour-piercing weapons. Cataphracts aren't quite as useful in this mod, and elephants have been nerfed a fair bit, making them very vulnerable to javelins. Archery in general has been nerfed greatly, and even levies have extremely good missle resistance from the front. Horse archers are still deadly in the hands of a human player, but unless you're playing as the Sauromatae, you'll find your enemies to be more than capable of putting up stiff resistance.

5): I'm fairly sure Carthage has a higher base growth than Rome.