I too agree with AnnaMagnani, and I recognise the pattern and escalation you've described here over the last month or so.
The reason you've never seen the aggression before, or this extreme escalation is because you've never stood up to her before. This is the first time her usual tactics haven't worked and she's had to deploy more extreme ones. So it's understandable too that you're struggling to stand your ground and keep getting dragged back in to her games.
Hard as it is, I think you need to accept that this is classic abusive behaviour and that as long as you continue responding to it, engaging with her arguments, and trying to appease her, it will continue.
The cycle of abuse that's playing out here: you said no to her and took her control away -> she went on the attack to bring you back in line with her usual tactics -> you continued saying no -> she ramped up her tactics to a more extreme level -> you wobbled ("we'll talk about it") -> nicely nice reappeared to enforce that as her desired outcome -> you still didn't cave -> massive escalation.
She will keep going until she gets control back. That is all that is playing out here, and I can imagine it must be incredibly distressing to realise she cares so little for your children she is prepared to hurt them by trying to use them as a weapon to get her control back.
Her actions are not your responsibility. Although I understand you will have been conditioned your whole life to believe otherwise.
When she used to call every 4 or 5 days saying how depressed she was for lack of contact, that was a control tactic. It made you feel bad so you ran around trying to appease her.
When she went out of contact for 2 weeks that was also a control tactic. She knew it would distress you and bargained on it pushing you into running after her again.
Sadly, by doing the latter she has exposed the former as nothing more than manipulation.
You are clearly a kind and caring person or you would not be so affected by all this - and she knows it and is trying to weaponise that.
Step back a little bit and look at what this behaviour reveals - label each time she has deployed a controlling tactic. It's hard but it will help you see what's going on and why you can't fix it.
The only thing that will make this stop is if you capitulate. That is the only thing that will send the cycle of abuse back to the beginning where she goes back to nicely nice mode - you had a brief moment of that from her because she thought she'd achieved control again.
You can't stop it, but you can stop feeding it.
She is not interested in reason, she is not interested in compassion, she is flat out not interested in anything other than her own wants. You could reason with her for the rest of your life and you would still make no headway, because she's not interested. Getting you to provide your reasons is a way for her to prolong the manipulation, to try and make you feel you've imagined things, to try and make you question your judgement, and to try and exhaust you so you break and give in.
Her behaviour is not a reflection on you, it's a reflection on herself.
My advice would be not to chase her - she will interpret it as you weakening and a sign her tactics are working, so she will continue.
If she makes contact or raises any of these points or demands reasons - do not engage. Find a one line response that you are happy with and repeat it calmly over and over. If necessary, end the call (the 3 strikes suggestion was a good one) or tell her to leave or leave yourself. "I've given my reasons and I'm not discussing this again" kind of thing.
If it transpires that she cares so little she doesn't make contact again, then sad as that is it does also confirm that none of this has been about her desperation to have her grandchildren in her life and have a positive relationship with them - it has been entirely about having control of them and you, irrespective of the risk or harm caused.
You are not a cow, you've just been conditioned to believe you are if you don't do what she wants.
If the roles were reversed, what would it take for someone to "make" you be aggressive, behave like this, or do what she did to your DD? I suspect there is nothing anyone could do to make you do any of this to another person.
Your caring nature is being exploited.
When you feel yourself being drawn in and your distress bubbling up, pause, take a breath, step back and label the tactic she has deployed, hold back from engaging - remind yourself "grey rock" if it helps. Keep breathing and walk away. Focus on why you are doing this - the welfare and safety of your children.
I'm really glad you've sought out counselling. That's a really positive step for the long term, although I too think you will need more than 5 sessions. For now though it is a great start.
You are doing the right thing. Start labelling her behaviours as the control tactics they are, stop engaging with them. Stay strong. You are doing what you need to do to protect your children, and yourself.
You can and will get through this.