the more attached they are, the easier it is to form attachments to the potential adoptive parents. I'm suggesting it would work exactly the same in reverse.
I think you may be seriously misunderstanding this. Yes, you are quite right that when a looked after child has bonded with a primary caregiver, this is considered a good sign for their ability to then form attachments to adoptive parents. But this is only in comparison to a child who has not been able to attach properly to any primary caregiver, NOT in comparison to children who have bonded with their parents in the normal way.
The really bad news is the baby or child who has never been able to trust and expect responsive, loving care, who has withdrawn into themselves because they profoundly distrust the world around them. Whereas a child who has attached will doubtless suffer trauma when that bond is disrupted, but having done it once they are more likely to do it again.
More likely, but no guarantees. And the more often children lose parents and the loving bond they have with them, the more likely they will stop trusting or expecting loving care, and will withdraw from attachments. So you do not multiply the protective factor of attachment by repeated disruptions: you multiply the trauma of it.
Do you see? There is no 'working in reverse', just more trauma to overcome, and reduced capacity for doing so. You can't dismiss this as 'much smaller' 'issues' - this is a massively big deal. I'm still not saying it is a bigger deal than being forced to live apart from your biological parents, but it might be. That is why none of us can know what the right thing to do here is. You are, simply, wrong to just dismiss this as insignificant.