Daughter-in-law can’t stop rushing around and finding things to.

I understand why you are concerned but your daughter in law has seen a gp and been prescribed medication and therapy. This will take time to work but it’s a good first step.
Try not to get too involved if you can. Just be there to help when/if asked. Hope she begins to feel better soon.

Can you invite your daughter in law to bring your grandchild to visit you? Or, if you're able, to sometimes get her to go out with you? Perhaps an excuse would be to get her to take you shopping for something that you need.

That would get her out of the house sometimes, give your son a break, and possibly help to break the cycle of activity that she has got herself into. It might be useful to try this once she starts showing a bit of improvement.

Assuming that it's not her usual personality, it sounds like a postnatal obsessional behaviour. The CBT should help - as would anything that teaches relaxation, such as yoga. It can take a long time to get back to 'normal' after childbirth.

Her behaviour could be due to overactive thyroid, which can sometimes be triggered by pregnancy and childbirth. When mine was overactive, I tended to rush around doing lots of stuff, then get exhausted but couldn't sleep - heart pounding and brain racing. I'd also be very jumpy - standing joke in family how easy they could give me a fright!

Sometimes thyroid hormone levels are checked by GP, but not always. If you get a chance, you could casually ask about blood test for thyroid.

Manic depression or bipolar will not show up on a brain scan
Pregnancy can change lots of levels in our bodies

Pulling all the plants up that she planted the day before and
then will bake the same cake several times to get it perfect
Sound like very obsessive and manic behaviour and I think the doctor is going down the right route to look at her mental health .
It doesn’t sound anything to do with not bonding

I think your daughter in law's behaviour is clearly getting out of control and affecting both her life and your son' Thomas I don't know how far along she is with her treatment but if things don't start to improve I think you should get more help for her. There are several conditions that can produce these behaviours as well as depression. I hope your family gets some support with this.

New mums have a lot to do. Naturally d.i.l wants perfection. Son bringing his daughter out once to a week sounds normal father behavior and not lack of mum to child bonding. Her doctor is working with her, maybe give time for antidepressants to work.

I'm sorry, I have no advice, but just wanted to say that your kindly concern does you credit.

I worked for somebody who suffered an episode like this. Up till then she had been a much respected boss, but her behaviour gradually became more and more irrational.

Trying to point that out or modify it some some way, trying to understand why the behaviour happens is impossible. At the moment her mind is working completely differently to yours, mine or most people. You cant understand it because it’s not comprehensible.

My ex boss, now thankfully fully recovered, describes it as living on a totally different planet where other people were the ones who didn’t make sense and only she could see what needed to be done so she had to do it.

Causes like a brain tumour have been ruled out with the scan and help has been put in place but it may be a long haul.

The best thing you can do is offer your son and grandchild all the support you can by making a safe space for them in your home, perhaps more than once a week if you can manage it.
If he could rent an office space so that he isn’t there all the time that might help his stress level or even come to work at yours, as long as he feels she is safe to be left.

It is so difficult I know when somebody is ill like this and you are involved with them all day, watching them and often being drawn in to their actions. And I didn’t have an emotional family involvement with my boss. At the end of the day I could walk away from it.

You might like to look up “Postpartum psychosis” on the NHS website.
Admittedly the baby is one and not a newborn, but a good friend suffered from this when she and I had our first babies and much of what you say resonates with me.