Sorry Colliewoman I have no experience of this, only ones which eventually come away of their own accord. When you say she is trailing the placenta, how much is hanging out? Could it be caught up on the wool rather than mostly being still in the uterus?
I would be very worried about going in to scrape it out manually, with no experience and no idea of what's actually going on in there, in case it leads to heavy bleeding. Perhaps the vet is thinking that it has all detached inside ok but just not been expelled.