save your game before trying it. If it goes bad, re-load the save and either kill the dragon first, or do something somewhere else so it doesnt spawn.
I feel for such well-manered people with seemingly unsolvable problems. Sorry buddy, can′t help you.
It′s absolutely not "your fault for not keeping more recent saves", we can never predict when the game is going to bug. You could keep more saves and have a pleasant surprise now, but that shouldn′t be strictly necessary.
Unfortunately, it is a huge game to code.
You should have an autosave before you conjure it because you have to go through a loading screen to get to that place. Just load that save (as you exit the Arcaneum), kill the dragon and then proceed with the experiment.
I only keep one hard save per character and use 3 autosave slots. In case my game crash (usually after 2-3 hours), i load the last autosave. Sometimes the game crashes during a loading screen, or i get an infinite loading screen. In that case, the last autosave is corrupted, but i still have 2 working autosaves.
This is not just a problem with this particular instance,.. at anytime if one "event" occur's over another, one of the two sometimes end's up glitched. This is why save's are very important.
I had a dragon attack as Meridia was telling my character about the going to get the orb... revert to a save leaving Solitude.. got to the Statue again...saved right there...and 4 tries later a dragon didn't attack and Meridia finished her spiel...
What I am saying is that its a product of the random encounter/ spawn timer. Revert to a previous save... and when you get to the same point.. save.. kill the dragon 1st then summon the Dremora. If you know it's going to happen, it's easy to circumvent.
That's a bummer...
I did look up some issues with this, but having done the quest...multiple times, I myself never ran into the glitch.... Most of the glitches seemed to be caused by one of two things... Not letting the Dremora finish his spiel or like you a Dragon interrupting the script. Either way the only "fix" seem's to be reverting to a previous save.
I don't know that the scenario can be "fixed" as it's caused by the Dragon's spawn timer.... and even if it can, they are not going to, it's unfortunate, but 5 yr's later, it just isn't going to happen.