[Maédhros]

Denying Melkor's release after promising it would indeed be wrong. They should never have given Melkor any sort of promise of getting out at all.

Again, I have to totally disagree with this.
From Ósanwe-kenta

Manwë could not by duress attempt to compel Melkor to reveal his thought and purposes, or (if he used words) to speak the truth. If he spoke and said: this is true, he must be believed until proved false; if he said: this I will do, as you bid, he must be allowed the opportunity to fulfill his promise.
The force and restraint that were used upon Melkor by the united power of all the Valar, were not used to extort confession (which was needless); nor to compel him to reveal his thought (which was unlawful, even if not vain). He was made captive as a punishment for his evil deeds, under the authority of the King. So we may say; but it were better said that he was deprived for a term, fixed by promise, of his power to act, so that he might halt and consider himself, and have thus the only chance that mercy could contrive of repentance and amendment. For the healing of Arda indeed, but for his own healing also. Melkor had the right to exist, and the right to act and use his powers. Manwe had the authority to rule and to order the world, so far as he could, for the well-being of the Eruhíni; but if Melkor would repent and return to the allegiance of Eru, he must be given his freedom again. He could not be enslaved, or denied his part. The office of the Elder King was to retain all his subjects in the allegiance of Eru, or to bring them back to it, and in that allegiance to leave them free.