Pretty straightforward build - as many spirits and arcane as possible, to reliably get whatever aura is best for neutralizing the enemy commanders. Hopefully that'll force them to use their own removal to recover their commanders, which with the protective auras should keep Tallowisp alive long enough to get it well protected with auras, and then drop some offensive auras - usually empyrial armor and/or ethereal armor, possibly with something to give the 'wisp flying if protection isn't enough to get it through.
Doubt it'll be as explosive as symbiote, but it has a lot more disruptive capabilities. Should be a fun addition to the old commander box.
Looks pretty similar to my list. I didn't think about mortal obstinacy, that one's actaully really handy to have since it gives you a whole different type of removal through tallowisp.
although there's a few cards in your list I'm curious about. Are the flagbearers worth it? I like spellskite, but the fact that they MUST target the flagbearer means that someone might HAVE to kill your flagbearer before targeting someone else's threat. So it could actually be a hindrance, unless you're the one with the most dangerous creatures.
I also have to say that I disagree with not including the other arcane/spirit instants. While they're not especially powerful (although quiet purity is fine), I think the ability to reliably tutor for an aura at instant speed (especially, say, cho-manno's blessing or temporal isolation) is worth it, and they're all cheap and instants, so for 3 mana you can protect your creature or neutralize theirs, and get the minor benefit of a defense boost or a token in addition.