the question is how much Homeworld already knows about Steven, between Lapis telling them about her time on earth and Peridot "reporting this" to the diamonds when Steven and the Gems confronted her at the Kindergarten
and I'm not sure if that's possible, it's been established that Rose's healing powers had limits and that Steven's are the same as hers, from what we know we can assume that a crack can be healed but a full break can't
they also can't heal corruption and at last check Steven and the CG's had fully given up on ever finding a cure for corruption