[Oh, hm. Yuya had a suspicion of the second part, but hearing it confirmed...]

Steven said The Rose Queen's making what it did to him worse. His arms are become crystals and--and there's roots and vines sticking out of him. She told him it was a good thing and she needed it.

So I think she's trying to use him to get the Blue Rose. [There's still a lot of 'hows' in there, but it makes her possession of Steven a lot more dangerous than just wanting her fake son.]