Teirae, your wall sounds beautiful!! I so wanna pinch the idea lol. Saw your Pinterest wall and LOVE the northern lights one - that's one that you can really just smudge along as you go, it'd be harder to make mistakes with it because it's not so definite. But yeah, not so space-y.

You can definitely get chalk-line from hardware stores, we've used it plenty of times for fencing and bordering our yard and stuff. If you use pencil to draw your lines on the wall, it depends on your paint and the colour as to how it'll be covered, and when you erase it, it can take the paint with it (learnt that the hard way!).

The house we moved into already had a nursery painted, the whole room is green with a jungle theme, one wall has an elephant, lion, giraffe, snake and butterfly painted on it. Beautiful It looks as though the paint used was just water colour, and it was hand painted. When you're up close you can pick out little mistakes if you try - like the elephant's trunk colour goes outside the line etc. But from the door, or if you're just generally looking, you don't notice anything except how cool it looks I reckon people (and Spock especially) will adore whatever you do.

Good luck!