I'm not debating the conclusion that "Ebony is a material formed from cooled lava". Actually I'm trying to come up with a reasoning that supports that conclusion. Yet I'm having a little trouble doing this from the facts you provided.
You use the word "Daedric" in a sense that seems to imply "Daedric ore". Yet Daedric ore is not a known concept in lore.
I didn't mean "Daedric ore", rather I meant ebony ore that is used for the production of Daedric equipment.
Yet we don't even have such a correlation. What I'd say is missing from the argument is the observation that raw ebony is found in places where lava is or once was in the Deathlands. Then we could say that raw ebony is (nearly) always found near lava so it might be cooled lava. Currently the only ebony found in the Deathlands is generally found in processed form near or on dead Dremora. Basically you need to find an ebony mine in the Deathlands.
It's not something with solid proof, it's rather something that with what we know, it's the most most probable explanation. Chances are, since Daedric is a substance so similar to ebony that it's commonly considered the same substance, it's formed in the same way. While http://www.uesp.net/wiki/Oblivion:Mehrunes_Razor#Other the case, it's most likely with what is known.
As for ebony mines in the Deadlands, or any sort of mine in the Deadlands, I doubt they'd be easy to find from Tamriel; none of the Oblivion gates allowed one to get too far in the Deadlands. And really, there's no need to put such a facility within the reach of the enemy.
I'm not trying to prove that ebony is indisputably igneous ore both in the Deadlands and in Vvardenfell, but show that it's the most likely and most feasible explanation.
Edit: I understand that there's some confusion in what I'm trying to say. I'll try to lay it out.
1) http://www.elderscrolls.com/games/morrowind_weapons_02.htm
1a) http://en.wikipedia.org/wiki/Volcanic_glass
2) http://www.elderscrolls.com/games/morrowind_weapons_02.htm
2a) Therefore because of 2 and 1a, Daedric equipment is made from an igneous substance.
4) The Divine barriers prevent opening up two-way gateways between Tamriel and the Deadlands.
4a) Therefore because of 3, with the exception of the duration of the Oblivion Crisis, the Dremora cannot mine ebony in Tamriel and ship it all to the Deadlands.
There are many possible explanations as to how the Dremora have so much ebony despite the barriers. They could mine it from another realm or engage in trade with Daedra of another plane. Perhaps when the Dunmer worshiped Mehrunes Dagon he demanded they send him large amounts of ebony. Perhaps some Daedric equipment, such as the Daedric Akaviri weapons, aren't made in the Deadlands but on Nirn. It's even possible they transmute other materials into it. But given what we know about (which, admittedly, is painfully limited) the most likely explanation is that they are able to harvest volcanic glass from the Deadlands, as there's no shortage of igneous material in a world of lava oceans.