the ringing 18 times thing....did your DH ignore her meaning she was trying to get through? Because if that's the case, he can hardly complain about her not letting him speak on the phone to the children. Playing tit for tat isn't the way to go.
Why could he not get his own copies of birth certificates if he needed them? Why would he need passport details, unless planning a holiday?
It is not unreasonable that clothes are returned ASAP. I don't buy my children clothes for them to sit on the floor at dad's house. I quite like to see them actually in them. Send them back, without fail, no stress for the children.
Re: photos and other school stuff. Was the paperwork for them removed from their bags? Was there an expectation that she made her selection through your DH? That can be solved by photocopying the form and letting her get on with it.
If unknown people are messaging you, ignore them. If they persist, remind them you are not friends and say any further contact will result in a report to police for harassment. And then follow through.
Last year's clothing as a general rule is too small for children, surely? Unless it was purposely bought big to last two years?
I personally have no desire to meet any of my exs partners and will never do so. There is no unfinished business, nothing to do with not making an effort, it is just not something I feel the need to do. I would be very dubious about any woman who felt the need to push herself on me, and I have no desire to be told that there is no attempt to replace me or my children will be loved. It's utter bollox in my opinion and is just trying too hard. My ex is capable of making decisions about who should be around the children and if the Children ever have an issue, I will take it up with the ex, not his partner.
I laugh out loud at the idea that any of this is parental alienation. It sounds more like an inability to hold firm boundaries and recognise that there are two sides to every story. Just because you think things should be done one way, doesn't mean a different way is wrong.