If he finds out how to rotate his gem, I have no doubt that he can convince the other Diamonds that he has Pink's gem. He has shape shifted parts of his body before, like in "Steven's Birthday". Theoretically, it should be no trouble to change the gem's orientation. When he relaxes it should sit nicely without going back. (I'm sure it will be safer too, without that big 'ol point pointing in his organs)
Now if he does prove he has Pink's gem, getting the Quartzes back is a matter of convincing Yellow Diamond. Now that it's clear that she was not shattered by a Rose Quartz, they're off the hook, and the question won't be if they're a risk, but probably if Steven will be responsible with them.
And will Steven be responsible FOR them. Remember that, as far as we know, no real Rose Quartz Gems have ever actually formed. They might turn out to not be at all like Pink Diamond pretended to be. They might look entirely different and have villanous tendencies. That may not be likely, but the writers could make them that way if Rebecca Sugar wanted them to.