OP this is just some information for you.
If Women's Aid are saying you need to live somewhere KH doesn't know where you are, that's them saying they'll help you get rehoused. It means you're effectively homeless despite co-owning a property and being given temporary shelter with PIL. Homelessness, especially with DC and fleeing DV is pretty much a golden ticket for getting a council place. I don't know if you could go down that route from PIL house. Maybe not, because if you turned down a hostel to stay with PIL it might be seen as you accepting that KH knowing where you live is fine, which would mean there's no grounds for homelessness. Speak to Women's Aid about the situation in more depth to ensure you fully understand all your options and the implications of agreeing/not agreeing. I realize there's also the issue of you having to pay for hostel accommodation, so it's not an easy decision, but UC exists and private rentals aren't cheap either. In the long run at least social housing is a secure tenancy that would give you all stability after the initial homelessness part.
If you follow the homelessness route you'd get rehoused in a new area I expect, to reduce the chances of KH finding you if he went looking. This also means you'd have an obvious reason, relocation, for leaving your job. Which might eventually become necessary if they're behaving like it's your fault KH accessed your work computer. I don't know how they can say that, but whatever, it doesn't look like they're sympathetic to the situation and employment tribunals are expensive and stressful, which you don't need right now. So although you don't want to go into a hostel, please think about the long term, including your earning potential to afford private rental if needed.
Speak to your solicitor about the housing issue too. I know you might be granted the family home in a divorce settlement but equally divorce can take years. If you need to live elsewhere for safety, then a house you can't live in, and can't sell either, isn't a lot of use to you at the present time. Nobody expects you to justify your decisions on here, I'm just asking you to take advice from the various sources available to you and think things through in your own mind to be sure you're happy with your choices. KH is arrested at the moment, but he'll soon be out again and he sounds dangerous.
I'm glad social services have accepted this and are in agreement he shouldn't see DC. If KH comes anywhere near you call police immediately, no giving him a chance to speak or anything else. You can't be seen to be putting DC at risk by having anything to do with him.
As awful as this sounds, you can't trust MIL, she isn't truly accepting the situation and if you had a new phone number or address there's a chance she'd give it to KH so be careful what you tell her. Which means also being careful what you tell to anyone who might pass sensitive information onto MIL.