Just learned there's 2 versions of various items and champion passives/abilities on aram. Why?

This is bad design IMO. For people that play casually, you're now expecting players to know that some items and champion abilities are different on different maps. So when you see someone going ROA on aram, your first instinct shouldn't be "This guy's not really thinking, that's a bad item on this map since it will likely spike too late." No, you should instead suspect that the item itself is different even though it has the same exact picture and costs the same.

Or take Nasus' passive on the map. He gets double the stacks for each minion. How is any player expected to know this?

Listen, if you wanted to make AD Nasus more viable, how about instead of changing how the champion works with some hidden stat you have multiple cannon minions spawn at each wave. That way he can get stacks much more quickly and there's no new random hidden stats introduced.

Or if you want to buff ROA and other time-based items, you just make it so the game clock is faster on that map (maybe you could explain it in the lore somehow, Zilean got raped there one time and put a curse on it or something), similar to how ambient gold accumulates faster. This would be much less hidden and require no reading or previous experience on the part of the player to figure out.