Hi Tenar. I don't have much relevant experience - I am an approved adopter but not matched yet - but I wanted to respond to your very thoughtful post.
You are right that we are not given much choice in agreeing to contact, but my understanding is that once the adoption order is final all final decisions are up to you, the parent. I have met adopters who feel very strongly that continued indirect contact was distressing and disrupting their children, and decided not to continue with it. I do think that parents should have the right to make this decision, once they can see how contact is affecting their child.
However, I did agree to contact (in principle) willingly (and I did tell them I was only agreeing in principle). I did so because I am anticipating a high possibility that any child I adopt will have a continuing interest in their birth family, or that they may become intensely interested during adolescence, and I want to be helpful to them in that. Maybe more honestly, I don't want to be blamed and the bad guy if they are furious that I haven't kept up contact! And I think contact may help them to reach a realistic, rounded view of their birth parents and why they were adopted.
I would be very, very unhappy, though, if the birth parents were writing inappropriately, e.g. telling my child they were coming to get them back, slagging me off, or in any way trying to disrupt the placement. And I absolutely would stop contact if I thought it was harming my child.
Your wider point about how we talk about birth families to adopted children is very interesting. In principle, I understand the honest-but-positive approach; in practice, I can see how this could be a struggle. I don't have any easy answers: how and when should you tell a child they were conceived through rape or abuse? That their dad used to beat up their mum so badly you had to be got out of there, and that their mum is still living in that situation? that their mum used to stub out cigarettes on them?
It sounds like you're further along this road than me. Are you matched? Is the contact issue a pressing current issue or something you are anticipating in advance? If the latter, could you not take it on a case-by-case basis, discuss your child's background and likely future needs with their social worker?