A few mechanicals quirks, or just my 2c.

Summon Seraph has a 120s cooldown, and Angelic Benediction has a 60s cooldown.
Since Angelic Benediction is only useable while Seraph is up, this makes Angelic Benediction's effective cooldown 120s. Unless this is intended to be on cooldown with the 3 other fairy abilities, which either also have long cooldowns (120s) or would require massive gauge (Feo Ul).

Why would you ever want to use Cancel Fey Deployment? As worded, it would put both fairies in play and lock their abilities out, while granting an extra heal on Physick/Science. There doesn't seem to be a reason to cancel it. The current Fey Union locks your fairy in place, locks it on a specific target and has a very short cooldown. It is cancellable because you might need to move it or change target. You do not have this problem with Fey Deployment.

Due to the way DoT snapshot in this game, Art of Poison is very poweful. This is because, as worded, the effect will compound:
Cyanide + Arsenic + Miasma II under raid buffs (lets assume a low 10%) = 126 per tick
126 * 1.5 = ~189 per tick (Art of Poison 1)
189 * 1.5 = ~284 per tick (After 2 minutes, Art of Poison 2, keeping DoTs active the whole time)
284 * 1.5 = ~426 per tick (Art of Poison 3)

The Science/Steel combo is pretty strange, because it would effectively be another movement tool. 15s duration translates into 3 Sciences and 3 Steels. Using 1 Science and 1 Steel is worse DPS-wise than one Broil III, but better than Miasma II and Ruin II. You wouldn't use it in heal-intensive situations because 'spamming' Adlo (making sure the shield breaks before-reapply) would be more effective, and you still get the same MP efficiency as Physick (1 MP = 1 heal potency).

As worded, it seems like you will always have enough gauge to use both Seraph and Feo Ul on cooldown. Embrace generate 5 per cast, 20 casts per minute for 100 gauge + 30 from Aetherflow actions gives 130 per minute. Seraph has a 2 minutes cooldown and takes 100 (50 per fairy) for 50 per minute. Feo Ul has a 3 minutes cooldown and takes 160 (80 per fairy) for ~54 per minute. 50 + 54 = 104 < 130.

Sorry if I got any of the math horribly wrong.