[Raynor]

Originally Posted by obloquy

It could be argued that corruption is what eventually bound Melkor, Sauron, and Balrogs to their material bodies, and weakened their spirits to the point that they could be rendered impotent by physical destruction.

While I admit this possibility, I also consider it is mainly their evil deeds which consume/ diminish their power. Concerning their vulnerability to physical destruction I would return to your previous observation that they are weakened by incarnation with this quote from Myths Transformed:

...by practising when embodied procreation they would (cf. Melian) [become] more and more earthbound, unable to return to spirit-state (even demon-form), until released by death (killing), and they would dwindle in force. When released they would, of course, like Sauron, be 'damned': i.e. reduced to impotence, infinitely recessive: still hating but unable more and more to make it effective physically (or would not a very dwindled dead Orc-state be a poltergeist?).

Originally Posted by obloquy

the result of the destruction of either would be the destruction of the other.

I believe the passage in question is this:

Originally Posted by Notes on motives in the Silmarillion, iii

The whole of 'Middle-earth' was Morgoth's Ring, though temporarily his attention was mainly upon the North-west. Unless swiftly successful, War against him might well end in reducing all Middle-earth to chaos, possibly even all Arda. It is easy to say: 'It was the task and function of the Elder King to govern Arda and make it possible for the Children of Eru to live in it unmolested.' But the dilemma of the Valar was this: Arda could only be liberated by a physical battle; but a probable result of such a battle was the irretrievable ruin of Arda.

In my interpretation, that the destruction Arda would be a result of war, not of the destruction of Melkor in itself.

Originally Posted by obloquy

Yet, in a late essay (within Myths Transformed, I believe), Melkor is said finally to have been executed as a mortal.

I agree:

Morgoth was thus actually made captive in physical form, and in that form taken as a mere criminal to Aman and delivered to NAmo Mandos as judge – and executioner. He was judged, and eventually taken out of the Blessed Realm and executed: that is killed like one of the Incarnates.

It is previously mentioned that this was possible because he became weakened and chained to his body.

Originally Posted by obloquy

His later evil was thus unnatural, while the primeval theme of discord he wove into the music can still be considered natural

I would speculate that his corruption was in direct connection with his part in the Ainulindale:

But even as Ulmo spoke, and while the Ainur were yet gazing upon this vision, it was taken away and hidden from their sight; and it seemed to them that in that moment they perceived a new thing, Darkness, which they had not known before except in thought.

Originally Posted by obloquy

Anyway, my point is that I don't think "Arda Marred" was a pool from which bad things received Melkor's lingering evil.

There is an interesting note in Myths Transformed concerning Sauron and Melkor's corruption :

Sauron, however, inherited the 'corruption' of Arda, and only spent his (much more limited) power on the Rings; for it was the creatures of earth, in their minds and wills, that he desired to dominate.

although I agree it hinges on whether corruption is a power to be used or a state of facts.

Originally Posted by obloquy

Still, were they not extraordinary?

Indeed!