Originally Posted by
Qualia
Was there any indication that he would have, much less did, figured the spell out on his own if Illidan didn't do anything? Malfurion was only trying to take the Dragon Soul from Deathwing so that the dragons would come back to help them again ("And they don’t dare do anything, not so long as Deathwing has the Demon Soul. (Rhonin) <...> "Then, we’ve got to take it from the black" <Malfurion>"). He didn't try to take it back for an awesome magic to revert the summoning magic and banish the demons back to the Nether. Later on, when they first met at the Well of Eternity, Illidan already started his magic towards the portal while Malfurion was still clueless, just trying to take the DS away from the portal's matrix. The idea didn't even cross Rhonin / Krasus' mind (at least, it hasn't been described so yet) and they both were a lot more knowledgeable in magic than Malfurion. It was only after he found Illidan that he started working against the portal, but everything they tried to do that didn't work out well. Malfurion figured out how to use the disc for their magic ("The Demon Soul is still part of the spellwork! Instead of drawing from disk, we should be giving to it" and it worked.
So, without Illidan's idea, was there any indication that Malfurion would have been able to suddenly figure that idea (using the DS to turn the Well into a portal that would suck the demons back to the Nether) out? Sargeras taking over the DS was unexpected, but it wasn't like Illidan couldn't use his spells away from the DS and the portal - he succeeded in casting spell into it before Malfurion retrieved the disc, after all.