Leo being brought back to Owen and Amelia seemed to me like a reflection of when Zola was brought back to Meredith, but it didn't feel as true to me. When Meredith opened the door to find Zola and the caseworker, I was in tears. Maybe some viewers expected that would happen, but I thought she was gone. Plus if I remember correctly, that episode had some wrenching events in it, so her return was especially startling.
I had a hard time believing the grandparents would give up Leo, although in reading some of your comments here, I find it easier to imagine. They want to devote time to their daughter, and their daughter does not seem interested in Leo. If she wants to give him up for adoption, they will abide by her wishes. (Reminds me of "Mom" in which the teenage daughter decided to let her baby go to adoption.) The about-face just seemed so abrupt, I guess.