Be careful with 1. The Warrior cost on Deity is placed almost perfectly to come online in time for when Barbarians start raiding cities. If the cost of the Warrior rises, and the player has to build a Warrior over infrastructure, when the AI has Warriors already and big barb combat bonuses, it will really tilt towards the AI in the early-game again, which we were trying to avoid. If anything, I think some of the early units (especially Spearmen) are maybe slightly too expensive for what they are. The problem starts kicking in around Renaissance, I wouldn't do anything before then.
I'd go with 3, and take it further. EXP bonuses only apply to built units full stop. Doesn't matter if you have a Barracks, if you purchase that unit, it doesn't get any XP. It's a mercenary unit from elsewhere, why would it get any experience from your barracks?
And no,
@Infixo, you're not right. Mercenaries were typically worse than professional armies - that's why mercenaries aren't used any more. If you look at the states which predominantly used mercenaries, such as the Italian city-states, they weren't chosen in preference to a domestic army, they were chosen because there wasn't the option for a domestic army - Florence was not going to be able to stand up to the German emperors in a straight fight purely on the disparity of the available manpower. If you look at the Eastern Roman Empire, they only started using mercenaries on a significant scale when they lost the eastern territories that provided the lands used to uphold their semi-feudal cavalry class. It was a sign of decline, not of strength. A professional standing army could be expected to be drilled together constantly and work fluidly as a unit, be provided and trained with standardized equipment, fit into a functional command structure, and was significantly less likely to defect - an age-old problem with mercenary armies. It's a real misconception that mercenaries were preferable to a standing army.
There was a brief period of history when mercenaries were particularly effective. European states used not to have the taxation to afford standing armies. Armies were raised for war and promptly disbanded afterwards. You had a semi-professional class of soldiers, knights, who were expected to remain familiar with the use of weaponry and the command structure, but how well that was actually understood was limited, since in reality they were rarely called. The rest of the army was filled in by peasant levies, who were largely unskilled. Mercenaries, by virtue of constant exposure to warfare, did tend to have an advantage on these 'amateur armies'; particularly the Swiss pikesmen because they were among the first forces to practice drilling with the pikes, whereas peasant pike units tended to scatter under pressure.
However, this period of advantage was a small window. It existed in the period where European states had enough revenue to commission mercenary forces frequently enough that mercenary units were profitable for mercenaries, but not so much revenue that they could finance their own standing armies. The decline of the mercenary era begins in the early 1500s, and you can see what happened when a standing army was pitted against mercenaries in the Battle of Ravenna (1512).
I can cite this with some literature if you like, I have tonnes of books on military warfare.
