Pretty much this. Tyrael never actually became human.
Tyrael, while appearing in a form similar to that of a human, is still considered an angel by those around him, be they friend or foe. The thing however is that Tyrael is now a mortal angel. As such, Tyrael can now presumably die of hunger, thirst, lack of sleep, illness, and among other things that can lay a mortal man to rest, most of which he didn’t have to concern himself with while he still retained his angelic form.
Also:
Assuming I’m reading this correctly.
Imperius didn’t remove Tyrael’s angelic essence; Tyrael had committed the act onto himself.
Now regarding Diablo:
It’s unknown whether Diablo, or any demon for that matter, can shed most of their demonic essence in order to gain mortality of sorts. Tbh, I don’t believe it would be possible, as even if they could remove said essence; there’d could be a rather problematic question that would need answer.
Where would that essence go?
Tyrael’s fall upon Sanctuary awoke all of the dead (those who were slain unjustly) within the area of New Tristram and then some. This was something that Tyrael couldn’t control. Now imagine if a demon of the lords of Hell calibur, were to just let loose their essence onto Sanctuary.
Demonic corruption isn’t something that can be removed (at least, no one thus far has shown the capabilities to do so from my knowledge); even Izual, whom the players supposedly redeemed during Diablo 2, was merely thrust back into Hell, only to be called upon by Diablo during Diablo 3. Also, taking a look at his dialogue during Diablo 3, Izual mentality seemed to have even welcomed the corruption.
In the end, neither demons nor angels can become human even if you purge their essence. You could try (if you had the tools and power) to strip them of most of their essence and then sealed said essence away; and thus leave them in a weakened state. However, as we learned from the events of the Diablo franchise, the more powerful the essence you try to contain, the greater it’ll leak out and eventually corrupt the area (as the soulstones and black soulstone did to their surrounding).