I don't have anyone super useful, but I wish I did! But I do have some people who might be good for random, fun threads.

For Myrtle:

I have three students. Unfortunately none are Gryffindors.
Archer Belby has an older sister who is a Gryffindor, and his mother is the Astronomy professor. He's a very painfully shy and lonely sort of boy. I could see him getting advice from Myrtle, but not actively seeking it out at first.
Fleur Rey is a Hufflepuff, who probably would just feed her cookies.
Thomas Montgomery is a Ravenclaw whose mother ran away with a pornographer rofl. So, if anyone in Gryffindor ever brought it up, he probably transfigured something they owned into something ridiculous.
Ava Gedge is not a student, but she's a woman who likes women! I'm not sure how they would stumble upon each other, but she's there.

I wish I had someone for Jack, but I don't. I have Ava who is working class. She works for a lawyer as his housekeeper/personal assistant. I'm not sure how they'd cross paths, though. (Also, Billie would likely idolize Jack, but it is highly unlikely they'd meet. haha)