The fact that in some jurisdictions a woman with capacity may be ordered to carry on with a pregnancy caused by rape has no relevance to this case.
Some do, some don't. The crucial point is choice
No, the crucial point is informed choice.
I understand that decisions are taken away from people who lack capacity in cases to look after their best interests but I really don't see how this is in her best interests when she blatantly objects to the procedure, adoption and one which I'm sure she would much prefer over the alternative
How can you possibly know this? All the indications are that she has no understanding that she can't keep the baby. Suppose she blatantly objects to adoption, should her wishes come first?
I fail to see how late term aborting of a 22 week old baby against the mentally disabled mother's wishes, whom already has a mood disorder prior to any of this, will be any less traumatic than a delivery under anesthetic and the baby which she has expressed a desire to keep
You have no idea what understanding this woman has of abortion and whether she would find it traumatic. My mother in law, who lacks capacity, had to go through a medical procedure which most people would find highly traumatic: she had no idea what was going on and sailed through it quite happily.
This woman may have expressed a desire to keep the baby, but one thing we do know is that she can't. So you want her to go through all the delights of late pregnancy and birth only to tell her that, although she did all of that because of her wish to keep the baby, actually she's not going to.
Perhaps your "fail to see" is because you haven't seen the papers, met the mother, seen and heard the witnesses, and heard all the legal arguments. Is it just possible that someone who has done all that may have a more reliable opinion than you have?