What happens if the baby is disabled? Would the couple still want it? What if it's twins and they only want one child?
All this is decided in the contract pre pregnancy.
Any woman selling parts of her body is vulnerable.
A baby is not part of a woman's body. The baby is in a separate area within a woman's body. And the baby is not being 'sold'.
How much screening is given to people who use surrogates
There is no assessment of their parenting skills. Social workers are involved in the parental order process to ensure that the surrogate is making a free choice and that she was not 'paid' (i.e. the fee is covering outgoings related in some way to the pregnancy). Social workers are also appointed by the court to look into the child's health and situation with the IPs. But they are not 'screened' as such and the child is already living with them at that point.
or that children aren't impacted by the loss/ removal of their birth families.
A baby being taken from it's mother isn't good for the baby amongst other things
You have the wrong end of the stick. Most children of surrogacy are not genetically related to the surrogate. They will always be related to at least one of the intended parents. So you are looking (usually) at the effect only of having a different gestational carrier (she will not appreciate you calling her 'mother'!). That is very different to adoption where genetics and often a period of time spent with mum is involved. The research underlines this difference. There isn't research to suggest that children are being harmed by surrogacy.
They will have to adopt to both be recognised
No, this isn't right. The legal partner of the person genetically related to the child is also the new legal parent (and the legal partner of the surrogate was the old legal parent, which is ridiculous).
You are deliberately creating a child to be separated from it's mother
See it that way if you wish. But the reality is that this is not a 'mother' in the sense that she is related to the child (usually) or has any inclination or desire to have a maternal bond. Experienced surrogates have no notion of having this bond and it simply isn't there to be available to the child. What is available to children of surrogacy is a couple of parents who are absolutely batty about them from the moment they are born. That is the reality for the children and it is a very happy, straightforward experience for most children.