Does Rabadon's Deathcap Scales off of AP Items Bought Before Or After?

its passive will increase your AP. It doesnt matter whether you bought the items before or after rabadoons, it even increases the AP you get from runes and masteries

Multiplies your AP by 1.3. No relation to items or anything, it just modifies the stat value.

My brain, it hurts...

Honestly it's a good choice for if you want to be aggressive. If you feel confident against your lane opponent, and feel that there is nothing they can do to respond to your intellect or your kit, then you should buy it first if that item applies because it will only snowball you harder.

However, when that happens is very rare. You have to learn to consider everything that could go wrong as well. What if the enemy jungler starts countering you? What if the enemy you're laning against will have an advantage when they get their ult? What if the enemy you're laning against scales better into the mid game than you do? If any of these answers are unsure, or for sure not in your favor, you should probably build a more defensive item first. That's just how I think about it.