Sparkmage

Can any of my spellcasters cast a spell and still benefit from my avatar's ability?

Yes! As long as you control the spellcaster, the game sees it as a spell 'you' cast, meaning the player.

Is the damage considered air damage?

Yes.

If I give my Sparkmage a poisonous dagger, will it deal lethal damage with its tap ability?

Yes, because it's dealing the damage directly via it's ability. (Damage from a lightning bolt cast by the Sparkmage would not be lethal.)

If I reduce the threshold required to cast the spell (e.g. Mix Aer), does the threshold still count for the Sparkmage ability?

Yes. The Sparkmage considers printed threshold on the .