I’m on console, so take my thoughts on Spirit Fox with a grain of salt, but it seems like a troop that would be a lot more useful in your hands than the AI’s. The value with the empowered mana drain is going to be mostly when you can target that to nip a problem in the bud. If the AI is just removing a bunch of yellow, things are as likely to go very wrong for you as anything. Any human player is going to fire-off any empowered troops they have before your Spirit Fox gets to do anything, which might force a sub-optimal Mercy cast or something, but otherwise doesn’t do you much good.
As for the others: For 3rd trait on CB, I think you’re right that it won’t do much good with that lineup. By the time it comes into play, you’re down to your last man. If you had CB up front (no point with EK), or a multi-bat lineup, that trait could be a lot more useful. For Venoxia, arcane is only going to increase the attack buff, which probably won’t make a lot of difference, and Miasma might add a bit of poison damage, which probably won’t make the difference between winning and losing very often.
You’ve definitely got me inspired to try some true-damage lineups in advance of EK’s introduction on console. I’ve got a team with Moloch and Saphira, but that team isn’t really about true damage. Overall, it’s an area where I’ve never really experimented much.