[The Saucepan Man]

Well, Gandalf knew that Bilbo had a ring that made him invisible and he didn't bother to find out more about it. I doesn't get any more clueless than that! Why did he have to wait all that time for?

As I recall, he had his suspicions from the outset. But, as he says, many Elven Rings were made, some more powerful than others. What were the chances of this being the One Ring? And he had to be sure before taking any precipitous action. Which is why he spent most of the intervening years trying to make sure. In the meantime, he couldn't risk taking it into his own keeping or trying to dispose of it. At least while it was with Bilbo, he knew where it was.

Also when Isildur took Sauron's ring, Elrond and Co. just said, well ok, and that was it.

What do you expect them to have done? Thrown Isildur into the fire? (There is a thread on that somewhere.)

I believe that the good guys won from sheer luck. They didn't have a brilliant strategy planned.

But as you imply yourself, their strategy was the only one that had any hope of succeeding. Not much hope, as Gandalf admits, but then he did put his trust in providence.