Good guys of LOTR clueless?

[Kuruharan]

There is something else to consider as well.

If Gandalf had been more vigilant, he could have just travelled to Minas Tirith circa 2942 to find out about the One Ring, so as to discard that it was not Sauron's ring, instead of waiting 75 years to do that. And a minor plus point is that probably Gandalf would have been better received by Denethor at an earlier time.

Gandalf went to Minas Tirith as soon as he had the idea that the answers he sought might be there.

Now I suppose he will be chided for not thinking of this sooner.

Regarding the search made for Isildur’s body…

I think there is evidence (already noted above) that Elves and Men tried to search for Isildur’s body. However, just because they did not find it is not proof of incompetence. Consider, they were operating in hostile territory. It is true that the local orcs had lost a good deal, but they were probably still capable of pouncing upon small parties of searchers. Later, when Saruman was searching, he probably had an understanding with the local orcs, if indeed they were not working with him. Elves and Men had no particular reason to guard the area, so Saruman was able to search the area most carefully and with security. Just because Saruman found the body does not mean that it was easy to find. It might have been that it took him a very long time to uncover it. We don’t know when the body was found. Saruman began searching in 2851. The next we hear of it is in 2939 when Sauron starts searching the area. That gives Saruman considerable time to drag an unlimited number of bodies from the river.