I have no problem with the Svartalfar, but what I dont see what the council of Esus does for them.
They are basically elves with +1 to recon rather than archers. WoF seems the obvious choice, they are going to want to research hunting as soon as possible, so WoF is just natural.
Having the AI prefer CoE just seems to be messing up the Svartalfar development for role-playing purposes. Well maybe thats ok, but I cant help feeling that they have somehow missed!?! Perhaps their Hero should need deception or something.
Been playing the Sandy, CoE seems much more natural for them. The Sidar seems to fit much better. Giving up all your best units is so painful!!
I was going to make a similar comment to this. The Svartalfar have the same massive synergy with Leaves that the Ljosalfar do, and it's hard to see Esus (or any other religion) ever being a better choice for them when Leaves offers so much. Perhaps even more than for the Ljosalfar, since their already-buffed hunters can also get poisoned blade from priests and become extra-deadly.
And I would only call them defensively weak in comparison to the Ljosalfar, who are pretty much the best defenders in the game. A carpet of ancient forest is an incredible defense against any attacker, particularly if you've got uber-recon units with woodsman promos to whittle down attacking stacks to nothing. They defend pretty well too, even in cities, since there's no anti-recon promotion. I'd have to consider them one of the strongest defensive civs in the game, using Leaves.
So I'd also consider them an excellent builder civ, based on their solid defense and the incredible tile bonuses that working forests gives.
Other problem with a big Leaves synergy: making them all lovey-dovey with the Ljosalfar!
I don't know exactly how it would be done, but I feel that the civ's design intentions (as well as its uniqueness from the Ljosalfar) could be better served by cutting down its synergy with Leaves. I know you can use Leaves and then switch to Esus after you're all forested, but that really feels horribly powergamey to me.
Suggestion right off the top of my head: their cottages can only grow to hamlets on forested tiles, and the forest needs to be chopped if you want to go beyond that.