Using CSS grid to align content in a non responsive div
I think the CSS looks fine. To your questions

a.) It doesn't matter which container has the grid property. It's CSS. It's not semantic.

b.) I am not sure if it is considered a best practice, but a lot of people seem to use it this way. Just google CSS nested grid and you will get quite some articles and videos on that topic. For example this and this.

c.) Counter question: Why not? CSS doesn't add any semantic meaning to HTML. It's just for styling. So, IMHO as long as it works and is scalable/maintainable it's fine.