I did some extensive testing tonight. Here is what steady does: it cancels the action in progress, but the cooldown timer keeps running its full length before the skill can be recast. It will only do this if you are changing stance from something else to steady. Once that cooldown is finished, Eos resumes auto casting as normal. Further uses of Steady will have no effect, unless her stance is switched first.

Attempting to cancel and recast embrace using steady will basically waste your fairy's cooldown for embrace without casting it, and then she will resume her auto casting.

Seravi, here is how I tested the responsiveness of embrace: I set up a simple macro that only does embrace, then jumped off a high building to land at 1 hp, and tried to make Eos heal herself or my chocobo instead of me. I found there was a very short window of time right when the cooldown is ending where I could sometimes get her to do what I was telling her to do. Even button mashing did not reliably produce the desired result, however. Neither did pressing the button right before cooldown ended.

From my testing, the only value I see in macros for embrace is that you can have her use it when no one is below 80%, instead of twiddling her thumbs. That and healing people outside your party (like in fates).

I also tested the percentage she heals at by positioning her at the bottom of a cliff, jumping off, and noting my health and whether she healed.
80.5% no
77.4% yes
Props on getting the percentage right, Seravi. I think I've seen 70% thrown around more often.