There is a document on the Guides page that has a more in-depth list on all the differences on the two versions if you want to see a more comprehensive analysis.

I don't know if it's been asked or addressed in previous posts, but ever since the site update, I've no idea on how to set up subcategories anymore. Adding variables just makes run variables, but I don't know how to change them into leaderboard subcategories. Can I still do that, or do I have to wait for some bug fixes to the site before that's available?

It might not run as well on the 360 as it does on an original Xbox, but we don't disallow it. Go ahead if that's what you've got.

Just make sure to tag your run with the appropriate console. I've added the 360 to the list, so you should be able to select it.

No reason why it can't be a thing. It would be interesting to watch, what with the levels being different for Co-op mode.