I'm also experiencing sibling jealousy at the moment (though not to your extent), so wanted to send huge
. I'm struggling enough just with the taking toys away, snatching, attention seeking etc., so really send you lots of sympathy.
Not got a solution, but just a practical suggestion for now that my HV gave me - could you put a travel cot up in your living room for now? With a playpen, there are bars that the older one can poke the little one through, but in a travel cot they are safer because they're off the ground and behind a mesh. Then at least you can just put your younger one in there when you have to go out of the room and you know that they are largely protected from the older one.
Even if you feel like you're neglecting Max a bit when he's in the travel cot, maybe you can just lavish attention on Harriet for a while. She'll have to be behaving better as she won't, hopefully, be able to get to him so you can 'catch her being good'. If he receives less attention for a few days but you can start to make things a bit better, it would be worth it in the longer term. As an odd idea (really don't know if it would work though), maybe you could then start to take Max out of the travel cot for short periods, with telling Harriet that if she then hurts him, she will be going into the travel cot for 5 minutes. Then gradually extend the periods of time that Max is out for.
Don't know if this will help at all, but hope things get better. It's a really horrible, difficult phase, but it is a phase and eventually, whether you find a 'solution' or not, it will pass.
Welly xx