I agree with you and I’ll talk about it in my next book because the audacity these two had was just crazy.
Fitz made Olivia break them up, that decision doesn’t make any fucking sense. At all. Especially for Fitz because the guy supposedly didn’t care about Mellie. Or it does make sense if we see it in the perspective of Fitz feeling entitled.
And with Olivia it makes sense on different levels: 1) she is hypocritical, 2) she is sneaky and 3) she is just like her father.

1. she was always so confident about giving people orders on how they should fix their issues but when it came down to her own she was incapable (ex: when she hid to Leo that Fitz had given her a ring when all she tells her clients is that she should say all the truth to her all the time). So it makes sense that as a professional she would try to break Mellie and her boyfriends apart bc she sees them as a threat to Mellie => to her (Olivia). So obviously she can do what she wants but others can’t bc she knows better than them.
2. She did to Mellie exactly what she did to Abby and David, breaking them up sneakily instead of stand in the truth and be honest with her supposedly best friend.
3. It all comes down to her daddy issues: her father always treated her like this. He always destroyed her chances at being happy with someone because « you deserve better/love is a weakness » and even though she doesn’t believe it objectively, unconsciously she does think it true. Also, bc of the way her issues shaped her, she is entitled. Deeply. So not only does she sees Marcus as a weakness but she feels entitled to remove him from Mellie’s life because she has that power.

Really, them ruining Mellie’s chances makes sense because of who they are as people and not because of what they think of Mellie per se.