...and Gandalf suddenly had a brilliant idea of how to win the war.

[the phantom]

and no phantom, I still don't consider the "diversion" at the Black Gate to be akin to a device of the Enemy.

The Ring could have been used to defeat Sauron...Tolkien makes it clear that it would have been wrong for the "West" to have done so.

No. In this case it had nothing to do with it being wrong to use a weapon of the enemy. It had to do with the fact that it wouldn't have worked. That's why it was wrong. If Sauron beats the new ring bearer, evil wins. If the new bearer wins, he becomes the next Dark Lord and once again evil wins.

The "cause" was to get rid of the Dark Lord. Using the Ring against him would ensure that there would continue to be a Dark Lord, so using the ring completely defeats the cause. That's why it was wrong.

If the ring had no corrupting powers and could be wielded safely there would be no problem with using its power to overthrow Sauron.