From my perspective, this is how the scene when Setsuna decide that they can take Marina back played out.

Setsuna did something out of character by showing some sympathy for someone other than Gundam.

Tiera comment was more of a surprised reaction to Setsuna unusual behavior rather than a disagreement.

Ian gave like a fatherly/elderly understanding to Setsuna request and goes off to prepare and his daughter follow his lead.

I think everyone just kinda agreed that Setsuna will be the one to decide what to do about Marina. It's not like they can just keep her there against her will or just randomly drop her somewhere.

Edit: The way Setsuna phrased his decision might make it sound more like a command rather than a request, but that doesn't seem to be the case here.