I'm curious, if Skill Drain negates Tyranno Infinity's effect, what would his ATK be? He dousn't have 0 ATK, it is undefined. Would it be treated as 0 anyway?

i believe so cuz if he cant get the effect of and thats how he gets the cards well he cant then get the cards therefore 0 atk

While on the field, monsters with an Original ATK/DEF of "?" will have an Original ATK/DEF of "0" if their effects are negated. In any other location, they do not have an Original ATK/DEF (which is why they can't be searched by Sangan/Witch of the Black Forest).