As far as exp goes, unless it was changed stealthily, then any units fighting should give XP to your sovereign, even if he isn't in battle. I used to see this all the time when I'd hire a bunch of champions and leave some for city defense, and they'd get leveled up as I fought around the map. I haven't gotten much playtime with 2A yet, though.
As far as mana goes, I agree that having your mana capacity linked to essence is a bit odd. My preference would actually be to link mana to shards. Each shard radiates a certain amount of magic that your Sovereign can tap into. The more shards in possession, the more magic is available to use - represented by mana.
I think overall, Essence has become a crutch for a lot of things that shouldn't have anything to do with essence. At first it was going to be this super stat that would make you extremely powerful individually if you hoarded it for your Sovereign, but would leave you weak in other areas (lack of cities, etc). But none of that is actually happening. Hoarding essence has no effect other than amount of mana, which doesn't make you more powerful and just allows you to cast more per turn. Essence gain is easy through leveling up. Only the first city really costs essence, provided you can wait for the green land to expand with your zone of influence and then just build the next city. It's also used in silly ways. Spending essence to help a wolf give birth? Why? Spending essence to teleport to city when you lose combat? Again, why? There are much better ways to handle that.
Essence is getting thrown around a bit too much, and it currently doesn't have any kind of effect that it was meant to. Maybe when the magic system is finalized it will improve, but it needs a lot of work.
- Essence should definitely be taken into account for personal stats. Otherwise it just doesn't work as a valuable statistic. You spend essence on your first city and.. your Sovereign is pretty much exactly the same as he was before short a few points of max mana.
- Essence should only be required for the most important things. Birthing wolves and teleporting after battle are not these things. Creating livable land is one. Giving a champion the gift of magic use is another. Casting extremely powerful world-altering spells is a third. These things matter.
- All changes in essense should have a noticeable and long term effect. Gaining or losing even 1 point of essense should matter both because it should have some effect on the Sovereign and because gaining essence should happen rarely. Founding your city, then killing a handful of silly spiders to get your essence back doesn't really work like that.