[Nerwen]

Yes, but... what can I say? It is very obvious indeed from the text that the "Lord of the Rings" is not Eru. Remember, the "full" title is The Downfall of the Lord of the Rings and the Return of the King.

Would Eru like to be referred to as the "The master of the Dark Tower of Mordor"? Are the Ringwraiths on his payroll?:

"...for the Black Riders are the Ringwraiths, the Nine Servants of the Lord of the Rings."

–Gandalf in Many Meetings.

If the Ring were sent into Tom Bombadil's keeping, would Eru react thus?:

"...soon or late the Lord of the Rings would learn of its hiding place and would bend all his power towards it? I think not. I think that in the end, if all else is conquered, Bombadil will fall, Last as he was First; and then Night will come."

–Glorfindel in The Council of Elrond.

I mean, the idea cited by ElanorGamgee is all very pretty and ingenious– until you look for evidence. Then your only way out is to decide that Tolkien was secretly writing a "God is Evil" story.

Or you could decide to shave with Occam's Razor, in which case:

–The title refers to the main villain, who despite never making a direct appearance, is absolutely vital to the plot; it's very much a bad guy-driven story. I mean, think about it: without Sauron, nothing in the story would have happened!

–It may, as you (Eönwë) say, also have the secondary meaning of "the struggle of who is the lord of the ring, or who wants it".

I think the confusion about the meaning of the title– of which there seemed to be a great deal when the movies first came out– arose because some of the new fans were perhaps fairly unsophisticated. Note the initial poster's shock at the idea of naming a story after the villain, which is actually not that unusual.