Why is it when I update an entry within a structure in the admin, it reorders it to the bottom of the list?
It's not intended behaviour when you're editing an existing entry, and it can't be changed - because it's not meant to be doing it. I noticed this on a site last week, but I can't replicate it right now.
I think it'd be worth filing a bug report on https://github.com/craftcms/cms/issues