For myself, I do think that the biggest error the Valar made "of old" was not taking care of Melkor and his threat
before the Elves awakened. At that point, the use of power (as in might) would have been appropriate, because that was how Melkor operated. He kept destroying things, Aule and others kept building them back up again. Manwe had been given the authority (as in power
) by Eru to deal with his renegade activities, since he had been appointed as the king, not Melkor (which I'm sure stuck in Melkor's craw). But he didn't exercise that authority to deal decisively with his brother, and the world suffered for it again and again. So I do not wonder that Manwe made another error in wanting to seclude the Elves rather than fight his brother to protect them. As I recall, Eru's specific annoyance with this was the lack of trust (or perhaps faith) in him that the Valar displayed, fearing more that a confrontation with Melkor would hurt the Children more than they trusted Eru to protect the Elves from harm. It really isn't just about having power, but knowing when its use is truly appropriate. I sometimes wonder if Manwe, in his naievete about Evil, also was in the dark about certain aspects of power -- ones that might "feel" evil to someone with that kind of innocence, whether they truly were or not. The Istari being sent in a diminished human form was an attempt, I think, to do it right for a change, and that still almost turned out to be too little too late.
Sometimes, it does make one wonder why Manwe was put in charge. Sometimes.
I will see about renting a bulldozer tomorrow. That room really is in a state, as it got crammed with all the junk from my last business when I dissolved the partnership. It was a messy, messy sitution, just like the room is now (which is probably no coincidence, either).