Granted. This does not mean that he's trustworthy, though. If anything, it makes him potentially more dangerous as a spy.Except Kyubey has shown sentience and the ability to talk, something the Devil's Rats never had. He's clearly not standard for the Devil's Rats.
We know that we were sent to "before she became the Devil", but we do not know whether or not the Devil's Rats report to her. She doesn't have ultimate power now, sure, but that doesn't mean that she's without spies.Also, the Devil has yet to awaken as the devil. We're trying to stop it from coming into existence.
Perhaps. But if you're talking about OOC knowledge, then Tatsuya would be also be adamantly opposed to beings who plan to turn his sister into a witch which destroys the solar system. IC or OOC, Kyuubey is not trustworthy.And OOC, Kyubey would likely be a very useful ally against Homurcifer if we explain the situation properly, since she apparently grabbed the whole universe, enslaved the incubators as mindless animals, turned part of Earth into a paradise, and left the rest of the universe to rot.