Life, this is a very rocky sea for you at the moment, and you must navigate it carefully. I am immensely sympathetic, but you must set some very clear guidelines for yourself and stick to them. And you will get through, quicker and with less collateral damage.
With email: either no response at all, or a very basic, functional, unemotional response. Just imagine she is reading it out to the police. Every single communication you have with him. Don't even say hi or bye.
For this email now: I'd suggest something along the lines of:
Thank you for enquiring about the children after 8 weeks of not responding to their personal messages to you. All is fine. I'm afraid, however, that a result of your lack of response up until now, neither child wants to communicate with you at the moment.
And that's it. Even that feels like too much emotion. But you can't avoid the truth.
With the OW: you need to understand that this woman has fashioned herself into your nemesis. This isn't in any way your fault - you are just the threat, and she's always going to want to take you out. Once she had claimed your ex for her own, she wanted to clear out the rubbish too - which is you. Understand that you are everything bad, and most likely the girls too.
Now this is a breathtakingly outrageous affront, and the sense of your ex giving this person preferential treatment just compounds it all into something too painful, hurtful and unfair to bear.
Yes. That's what it is.
Now pick that up and plonk it a bit away, where it won't bother you for now.
And focus on how you deal with an enemy. You don't give them anything. No information. No hint of your plans or feelings. No sight of what state you're in behind your shield. And definitely no chink, no way in to you.
Do not ever write to or otherwise communicate with or address that woman. Do not give her either the satisfaction or the ammunition or both. She has had enough of a field day with your and your children's lives already.
The same pretty much goes for him, but some communication with him is necessary, and he won't be as hostile as presumably does somewhere love his own children, and probably has some feelings of guilt/pity towards you. She doesn't have those feelings. She will be merciless.
And if you need to vent, write it down today but don't send it. Review it tomorrow and then you'll understand why you shouldn't send it. It will only backfire at you.
And now take what action you can, and make the most of the eerie silence of Lockdown.
Make an income plan. Face the reality. Can you get income from anything else? Can you do something? Money always helps.
Make a home plan. You need your own home. Do you need to sell or can you keep the house and him just be out of it? Investigate all options.
Work on yourself. Exercise. Get strong. Take vitamin D. Relax. Clear him out of your mind and soul.
Talk to the domestic abuse line(s) and ask them what to do about the insecurity of the house. And about everything.
I really feel for you. But the quicker you can face up to reality, distance yourself from the emotional melee and get strong, the quicker you and your girls will be out of the woods.