I think you're massively overreacting here.
You say the ex contracted him once a year ago, to talk about her daughter and work experience. And then a couple of months ago to do the same thing.
That's not a lot, and it seems purely in service of her thinking he can help her daughter launch into her preferred career. Maybe she thinks he's best placed to help over others, maybe she isn't as close as you believe to them, maybe he has a history of being helpful, who knows.
Unless there's tons you're not saying, and you need to elaborate on when/what the alleged "stalking" behaviour was (as in, was it actually, or was she maybe just trying to speak to him to get closure, which might be ill advised, but is common at the end of a relationship), then it doesn't sound like anything to worry about at all.
And given that they have to leave early and she's an hour away, and if the daughter is old enough to do work experience and both he and the ex know that he has people to stay regularly, and you have said yourself that he had a friend's teenage daughter to stay which you had no issues with, then to me this comes across purely as a matter of your insecurity and lack of trust, not that there's anything strange about it. Maybe she does want something, who can tell, but the requests she's currently making are not that, and your BF helping her daughter is not any indication that he wants her back, is undermining trust in your relationship or anything like that. That's all coming from your insecurity.
What is the alleged lack of honesty and transparent communication? It sounds like he's being perfectly open, you haven't mentioned any deception or withholding here?