[2021] Question about healing/shielding triggers

First, if I have Font of Life, and I hit 5 enemies simultaneously (eg Nasus Spirit Fire) , do all of them get fonted or just one, and if one, who exactly?

It won't be everyone, I believe we set it up to apply to your closest target.

Second, if I heal or shield everyone simultaneously (eg locket), do I get 4 shield triggers or one? I’m also curious how this interacts with Athene’s consuming blood.

You will get multiple triggers, but I am unsure how it interacts with Athenes. I would guess either gives the bonus healing to the lowest health target, or the closest.

Third, with the now 4 items I’m aware of that trigger off heal/shield, do you get multiple of their effects if you heal/shield multiple times, eg getting 40+ extra ability haste from staff of flowing waters.

Buffs with the same name do not stack. You will apply the buff to multiple allies though, but only gain one stack on yourself.

Fourth, with hextech megadrive, if I use locket on all 4 allies, how much of locket’s cooldown is restored, if at all?

It goes on cooldown after a single heal, meaning that even if you locket and hit 4 allies you will only get a single instance of the heal.

Last, is Athene’s unholy grail’s healing separate from other healing triggers, so if you shield and athene’s triggers, do you get two healing triggers for staff, hextech megadrive, or ardent censer

Athene's will only trigger once because of the cooldown.