The mandate of heaven says that it disables all decisions that changes my government type, while the Mongol empire decision says it destroys the mandate if it exists, even if I am the emperor?
That's basically a lie. It disable SOME decisions (people have said prussia gets disabled for instance), while some are not. You can form the MGE tag as the celestial empire, it will permanently eradicate the mandate. And change your gov, but that's incidental.
I'd recommend staying a horde if you plan to go for MGE.