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You cannot hide when you are agroed. Hydra does not lead to agro since you are installing a device that happens to hurt things; technically it is not an attack. This is why after plunking it you can re-hide. Similar to lance charge on a crystal deer. When you lance charge while mounted, YOU are agroed not the deer, so it can rehide, allowing you to rehide while agroed, which then cuts agro. Think of it as a lawyers approach to combat. XD
Technically aggro has nothing to do with hide besides the fact that it removes it; what does matter is whether you have initiated combat. Hydra works for reasons similar to how you've explained as well as lance charge, but it's combat that matters. Hide's useful because you use it to get rid of aggro. My favorite way of playing, since I rarely ever do, as an elf is to summon many pets that do on-summon damage (Pegasus, Bone Dragon, etc.) to deal damage (thereby not entering into combat myself). The best part about this technique is that it can be done while still inside of Hide, whereas using Hydra temporarily reveals you to danger.
I likewise have obtained the event quest despite having completed it before. It's worth noting that there doesn't seem to be any possible way to complete the quest, since Enn reacts as if the player has still completed the Momento sidequest (and therefore one cannot "complete" said questline).