Inlaws? Cousins? Aunts? Name terminology...

Not sure where you would need this but I'd say Grand Uncle/Aunt. Although Uncle/Aunt are fine. I still call my niece's daughter my niece.

I don't think there is a term for extended in-laws. I don't consider my sister's in-laws mine, and refuse to call her mother-in-law "mom" even though she wants me to.

I don't think there really is a name for that relation because it's not an actual relation. To your new niece, she would just refer to your parents as her "Aunt Whippppss' mom and dad."

If it helps any, my cousins on my dad's side always referred to my mom's parents by their first names or just called them the grandparent nickname I used for them. I think as we all got older, they have gravitated towards using their first names.

There really isnt a need to attach a title to it, either mr and mrs whatever or by their first name

When I grew up, "Aunt" and "Uncle" were often used for close friends of the family--too old to use their first names, but too close to be called "Mr. and Mrs." So if the children actually know them, that would be appropriate. But I could not name the parents of any of my aunts and uncles by marriage, so the question has never come up.

I have not been in this situation, but I call my great-aunts "aunt" and my dad's cousin my cousin, so I think she would be fine to just call your parents her aunt/uncle or cousin, either one, if she considers them like family.

Biologically only one person can call me "aunt," but a few of my friends' kids call me Aunt Stephanie/Tia Stephanie. Also I call various people aunt/uncle who aren't biologically related.

I think just aunt and uncle are fine. Maybe she will come up with a cute nickname for them.

No relation

They're not her family, so just their names are fine.