@Riot Why does illaoi's E give her so much?

Test of Spirit: ACTIVE: Illaoi sends a tendril in the target direction that stops on the first unit hit. If the unit is a champion, the tendril pulls their Spirit, granting True Sight of them and placing it in front of Illaoi. The Spirit is connected to the target by a tether. The Spirit has 100% of their current health and lasts for 10 seconds, reduced by 1 second each time Illaoi receives damage from the target champion. The tether will be severed if the Spirit is killed or when the target moves too far away. Over the duration, the Spirit can be attacked by Illaoi and her allies. A percentage of the damage dealt to the Spirit is also dealt to the the target. DAMAGE PERCENTAGE: 25 / 30 / 35 / 40 / 45% (+ 8% per 100 AD) Severing the tether Slow icon slows the target by 80% decaying over 2 seconds, and marks them as a Vessel Vessel, causing them to spawn a Prophet of an Elder God Tentacle every 10 seconds for the next 60 seconds. If the Vessel Vessel kills 3 Prophet of an Elder God Tentacles or scores a takedown on Illaoi, they are freed from their curse.

Why does one ability have the potential to cause 53%+ your current health, give true sight, an 80% slow, 30 seconds of free farm time for Illaoi, and if you have to destroy all the tentacles her ability is already off of cooldown. Why is this even allowed to be in the game?

Here are some of my ideas on how to maybe balance this ability, a little bit.

1. increase the cast time to be comparable to the time it takes Thresh to cast his Q.
2. if tether is broken or the spirit is killed within 10 seconds have all 3 tentacles spawn at once with additional tentacles spawning each 10 seconds after that.
3. Allow allies to kill tentacles to break the curse.
4. reduce the curse time from 60 seconds to 30 seconds.
5. remove the 80% slow, its just too much and over the top.

Why i think these are healthy changes:

1. Increasing the cast time I think just makes sense based on the potential this ability has.
2. spawning all 3 tentacles at once creates more of a hazard for the player hit, but it also allows to break the curse much more quickly reducing the time Illaoi gets to free farm.
3. If you get hit with this ability in lane with multiple allies, the tentacles still hurt them. The curse directly has an affect on them giving them the ability to break the curse really only makes sense.
4. if 3 isn't implemented, it takes a tentacle 10 second to spawn. any missed tentacle adds an additional 10 seconds of time you must essentially stay away from your allies as to reduce potential damage while putting you at risk of danger or loss of EXP.
5. the 80% slow is basically just overkill for everything else this ability does. Its just too much.

Edit: I'm not suggesting all changes, but I wanted to give examples of changes I felt could benefit game play.