Well Diz I think the high five and full on conversation options are rather scary. They could bomb badly if you're not a natural at pulling them off. Whatever you do has to be genuine and natural rather than forced, or else she will sense the awkwardness.

At this stage if you see her I would just smile and and say 'hey', and then just keep right on walking. You can't fuck that up and it's not something to get worked up about.

She's at work, so probably not on the pull. If she's on her phone a lot, it might be a boyfriend. Just get the first hello out the way before you do anything else.

Don't think beyond that. At least wait until you find out her name, and if she's even single before you try any high fives or imaginative efforts to seduce her.