Obviously any ONE of your five extremely strong reasons for not doing this is more than enough of a good reason. It would break up your marriage, it would cause you mental health issues, and it wouldn’t make anyone happy, including, actually, your Mum.
She has absolutely no right to be emotionally blackmailing you as she is. My gues is she has dine this all your life.
She won’t come round to your way of thinking, she won’t depart from her high drama ‘poor me’ position, just as she will always blame you for your PND. She will never take any responsibility, just as she is taking no responsibility for finding friends and activities to make her own life better.
So what you need to do is, in effect, exactly what she suggests: go on a course to make you feel hard!
Except being resilient to other people’s emotional abuse (because that is his she is behaving) isn’t ‘hard’ it is simply understanding the situation and not becoming sucked in as a victim.
She is a fit and able middle aged woman with a considerable financial asset. You do not need to feel sorry for her or take responsibility for her insatiable drama and negativity. (Read up on narcissistic personality disorder).
You have a view of s perfect ideal family where generations and extended family live in harmony an help each other out. But you need to come to terms with the fact that you do not have that family and you can’t magically turn your Mum into that Mum.
You have done a great job telling her no, and even being clear that it isn’t all your DH. You did really well there, those conversations are not easy.
Now all you need to do is rid yourself of the guilt that she has planted in you.
Good luck OP!