Why is it when I try and do a mass update on a setting (i.e. lead time), the values do not appear to stick? When I update them individually in options, it the change is immediately applied.
The reason you see your setting changes as you make them in the item options view is because the change is applied as you save them. From there you can review the impact and further change them prior to exploding the requirements to all its dependencies.
When you perform a mass update of a setting change, it applies the change the next time you run a new requirement plan. This is because when you view an item prior to running a new plan, it shows the settings as of the last time it was run so that it reflects the settings at that time. Once you run a new plan, the new settings will be applied and will be reflected when viewing the item detail.